feat(xml): add gradient support

examples/others/xml/my_card.xml

@@ -1,4 +1,12 @@
 <component>
+
+	<gradients>
+		<horizontal_gradient name="grad1" >
+			<stop color="#ff0000" frac="30%" opa="100%"/>
+			<stop color="#00ff00" frac="200" opa="100%"/>
+		</horizontal_gradient>
+	</gradients>
+
 	<consts>
 		<px name="size" value="100%"/>
 	</consts>
@@ -12,11 +20,11 @@
 	</api>
 
 	<styles>
-		<style name="gray" bg_color="0x888888"/>
+		<style name="gray" bg_grad="grad1"/>
 		<style name="blue" bg_color="0x0000ff"/>
 	</styles>
 
-	<view extends="lv_obj" style_radius="3" width="#size" height="content" style_bg_color="$bg_color" >
+	<view extends="lv_obj" style_radius="3" width="#size" height="content" styles="gray" style_bg_color="$bg_color" >
 		<lv_label text="$title" align="left_mid"/>
 		<my_button styles="$btn_rel_style $btn_pr_style:pressed" btn_text="$action" align="right_mid"/>
 	</view>

examples/others/xml/view.xml

@@ -10,7 +10,6 @@
 	</styles>
 
 	<view extends="lv_obj" width="280" height="content" style_bg_color="#light_blue">
-		<lv_label text="Hello"/>
 		<my_card title="Card 1"
 				 y="0"
 				 btn_rel_style="btn_style"