docs separate widgets into core and extra groups

docs/widgets/extra/tabview.md

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+```eval_rst
+.. include:: /header.rst 
+:github_url: |github_link_base|/widgets/tabview.md
+```
+
+# Tabview (lv_tabview)
+
+## Overview
+
+The Tab view object can be used to organize content in tabs.
+
+## Parts and Styles
+
+The Tab view object has several parts. The main is `LV_TABVIEW_PART_BG`. It a rectangle-like container which holds the other parts of the Tab view.
+
+On the background 2 important real parts are created:
+
+- `LV_TABVIEW_PART_BG_SCRL`: it's the scrollable part of [Page](/widgets/page). It holds the content of the tabs next to each other. The background of the Page is always transparent and can't be accessed externally.
+- `LV_TABVIEW_PART_TAB_BG`: The tab buttons which is a [Button matrix](/widgets/btnmatrix).
+Clicking on a button will scroll `LV_TABVIEW_PART_BG_SCRL` to the related tab's content. The tab buttons can be accessed via `LV_TABVIEW_PART_TAB_BTN`. When tabs are selected, the buttons are in the checked state, and can be styled using `LV_STATE_CHECKED`.
+The height of the tab's button matrix is calculated from the font height plus padding of the background's and the button's style. 
+
+All the listed parts supports the typical background style properties and padding.
+
+`LV_TABVIEW_PART_TAB_BG` has an additional real part, an indicator, called `LV_TABVIEW_PART_INDIC`.
+It's a thin rectangle-like object under the currently selected tab. When the tab view is animated to an other tab the indicator will be animated too.
+It can be styles using the typical background style properties. The *size* style property will set the its thickness.
+
+When a new tab is added a [Page](/widgets/page) is create for them on `LV_TABVIEW_PART_BG_SCRL` and a new button is added to `LV_TABVIEW_PART_TAB_BG` Button matrix.
+The created Pages can be used as normal Pages and they have the usual Page parts.
+
+## Usage
+
+### Adding tab
+
+New tabs can be added with `lv_tabview_add_tab(tabview, "Tab name")`. It will return with a pointer to a [Page](/widgets/page) object where the tab's content can be created.
+
+### Change tab
+
+To select a new tab you can:
+
+- Click on it on the Button matrix part
+- Slide
+- Use `lv_tabview_set_tab_act(tabview, id, LV_ANIM_ON/OFF)` function
+
+### Change tab's name
+
+To change the name (shown text of the underlying button matrix) of tab `id` during runtime the function `lv_tabview_set_tab_name(tabview, id, name)` can be used.
+
+### Tab button's position
+
+By default, the tab selector buttons are placed on the top of the Tab view. It can be changed with `lv_tabview_set_btns_pos(tabview, LV_TABVIEW_TAB_POS_TOP/BOTTOM/LEFT/RIGHT/NONE)`
+
+`LV_TABVIEW_TAB_POS_NONE` will hide the tabs.
+
+Note that, you can't change the tab position from top or bottom to left or right when tabs are already added.
+
+### Animation time
+
+The animation time is adjusted by `lv_tabview_set_anim_time(tabview, anim_time_ms)`. It is used when the new tab is loaded.
+
+### Scroll propagation
+
+As the tabs' content object is a Page it can receive scroll propagation from an other Page-like object.
+For example, if a text area is created on the tab's content and that Text area is scrolled but it reached the end the scroll can be propagated to the content Page.
+It can be enabled with `lv_page/textarea_set_scroll_propagation(obj, true)`.
+
+By default the tab's content Pages have enabled scroll propagation, therefore when they are scrolled horizontally the scroll is propagated to `LV_TABVIEW_PART_BG_SCRL` and this way the Pages will be scrolled.
+
+The manual sliding can be disabled with `lv_page_set_scroll_propagation(tab_page, false)`.
+
+## Events
+
+Besides the [Generic events](../overview/event.html#generic-events) the following [Special events](../overview/event.html#special-events) are sent by the Slider:
+
+- **LV_EVENT_VALUE_CHANGED** Sent when a new tab is selected by sliding or clicking the tab button
+
+Learn more about [Events](/overview/event).
+
+## Keys
+
+The following *Keys* are processed by the Tabview:
+
+- **LV_KEY_RIGHT/LEFT** Select a tab
+- **LV_KEY_ENTER** Change to the selected tab
+
+Learn more about [Keys](/overview/indev).
+
+## Example
+
+```eval_rst
+
+.. include:: /lv_examples/src/lv_ex_widgets/lv_ex_tabview/index.rst
+
+```
+
+## API
+
+```eval_rst
+
+.. doxygenfile:: lv_tabview.h
+  :project: lvgl
+
+```