BeginInit/EndInit breaks sometimes theme change
minor change to ControlsHelper
improve TabItem HeaderFontSize
default HeaderTemplate for GroupBox
TabControl left style in default GroupBox
use MetroTabItem in normal TabControl
improved TabItem/MetroTabItem

- it's now possible to have a border
some changes comes from #566
- Check if closeButton is not null when MetroTabItem is unloaded
- Closebutton in MetroTabItem collapsed insted of hidden when not used
use configurable font sizes for default button and ToggleButton
FlatButton's font size should equal content font size
consistent window title font size
ListViewScrollViewerStyle doesn't work
better MetroGridViewScrollViewerStyle
Merge pull request #743 from thoemmi/StyledFontSizes2

use configurable font sizes in styles
set FlatButtonFontSize to 14

looks much better than 12
change theme only if needed