minor documentation...
remove unused isDragging variable
rework the show system menu

show the system menu on
- icon
- if mouse yPos <= TitlebarHeight
- UseNoneWindowStyle = true
introduce ShowSystemMenuOnRightClick
remove more unused stuff
introduce ShowWindowButtonCommandsOnHiddenTitleBar
minor change to MetroBaseWindowButtonStyle

move content template triggers to style triggers, so we can better
override it
Merge pull request #1414 from MahApps/punker76-titlebar-enhancement-etc

titlebar enhancement etc...