[WPF] Widget allocation optimization

Box: Only measure the child widget when it is first added to the box.
[WPF] Better defaults for stepping increments in ScrollView
Merge remote-tracking branch 'origin/wpf-fixes'

Conflicts:
	Xwt.WPF/Xwt.WPFBackend/BoxBackend.cs
[WPF] Properly implement WindowFrame.Close