Fix: TextLayer is rendered behind SelectionLayer

[mgarstenauer]

Problem:
The TextLayer is rendered behind the SelectionLayer, which makes the text hard to read when selected. If you style the control and set an opaque SelectionBrush then the text is obscured completely.

Cause:
The problem is that TextView.Layers and TextView.VisualChildren are out of sync:
The Layers are

1. SelectionLayer
2. TextLayer
3. CaretLayer

but the VisualChildren are

1. TextLayer
2. SelectionLayer
3. CaretLayer

Fix:
The LayerCollection needs to insert the control at the correct index. (The first n visual children should correspond to the n layers in the LayerCollection.)

[danipen]

LGTM.