style formatting
respect font size (#471)
adapts Calendar button symbol to textbox height
use same hover color as textbox does
use ContentFontSize, ContentFontFamily

instead 10 and DefaultFont
SnapsToDevicePixels=True for Calendar
some style issues

- Possible to change font size used in calendar
- Calendar should use AccentSelectedColorBrush as foreground color for
the selected date
- Styling issues with bigger fontsize
fix not showing validation
use actualheight from inner grid for button width
Merge pull request #673 from MahApps/469-punker76-datepicker-issues

@spiritdead @Amrykid Don't mention it! :-)