completed extend-controls docs

doc/source/handycontrol/extend_controls/imageSelector/index.md

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+---
+title: ImageSelector 图片选择器
+---
+
+该控件封装了图片选择、展示、删除的一整套逻辑.
+
+```cs
+public class ImageSelector : Control
+```
+
+# 属性
+
+|属性|描述|默认值|备注|
+|-|-|-|-|
+|Stretch|图片拉伸模式|Stretch.None||
+|Uri|图片资源|||
+|PreviewBrush|预览图画刷|||
+|StrokeThickness|边框粗细|||
+|StrokeDashArray|边框虚线数组|||
+|DefaultExt|默认图片后缀名|.jpg||
+|Filter|图片过滤字符串|(.jpg)|*.jpg|(.png)|*.png||
+|HasValue|是否有值|false|||
+
+# 样式
+
+|样式|描述|
+|-|-|
+|ImageSelectorBaseStyle|默认样式|
+
+# 案例
+
+```xml
+<UniformGrid Columns="3" Margin="16">
+    <hc:ImageSelector Width="100" Height="100" Margin="16"/>
+    <hc:ImageSelector Width="100" Height="100" Margin="16" hc:BorderElement.CornerRadius="50"/>
+    <hc:ImageSelector Width="100" Height="100" Margin="16" hc:BorderElement.CornerRadius="50" StrokeDashArray="10,5"/>
+    <hc:ImageSelector Width="100" Height="100" Margin="16" hc:BorderElement.CornerRadius="50" BorderBrush="{DynamicResource SuccessBrush}"/>
+    <hc:ImageSelector Width="100" Height="100" Margin="16" hc:BorderElement.CornerRadius="50" StrokeDashArray="10,5,10" BorderBrush="{DynamicResource DangerBrush}"/>
+    <hc:ImageSelector Width="100" Height="100" Margin="16" hc:BorderElement.CornerRadius="10" StrokeThickness="2" BorderThickness="2" BorderBrush="{DynamicResource PrimaryBrush}"/>
+</UniformGrid>
+```
+
+![ImageSelector](https://raw.githubusercontent.com/HandyOrg/HandyOrgResource/master/HandyControl/Resources/ImageSelector.png)