Theme Chooser + PersistentData to file

*Added: The possibility to choose between Light and Dark Theme. View ->
Theme
*Added: Options and PersistentData classes, all options are save to
PersistentData.xml. Replaces skilladdress.txt
change border color
add another  row on mainwindow so buttons have a background
1. Merge remote-tracking branch 'upstream/master'

Conflicts:
	WPFSKillTree/Views/MainWindow.xaml
	WPFSKillTree/Views/MainWindow.xaml.cs

2. fixed an issue with "Download items" popup on light theme
udate search when mouse leave attribute list