Fix TextBlock constraint not getting initialized.
Moved a bit of stuff around.
Don't let TextBox collapse when it has no text.
Remove LayoutHelper.MeasureDecorator.

No longer needed as most of the logic was moved to MeasureCore.
Fixed easily-fixable failing tests.
Increased fail threshold for failing image tests.

The images were identical to my eye.
Assert inconclusive for unimplemented features.
Some work on Paths.

Make paths with Stretch != None correctly fill the Path control's
available size. Added a Path_Tick_Scaled test with a path from
MahApps.Metro to test this.
Improve CheckBox style.

Give it a proper tick (taken from MahApps.Metro) and improve the
spacing.
Center the CheckBox label.
Fix potential reference after dispose.
Adjusted triangle output again.
Added RadioButton control.
Added contribution guidelines.
Fixed punctuation.
Merge branch 'fixed_letterA'