Update Icons.xaml

Added the following icons:
appbar_bold
appbar_italic
appbar_underline
appbar_font
appbar_fontsize
appbar_fontsize_increase
appbar_fontsize_decrease
appbar_textcolor
appbar_marker
Update TransitioningContentControl.cs
Update MainWindow.xaml
Update MainWindow.xaml.cs
Merge remote-tracking branch 'MetroBehind/master'
Indentation fixed

Strange, I thought everything was indented correctly...
Merge pull request #910 from Dotnetifier/master

Added icons and added TransitioningContentControl transition feature