minor changes to slider style

- add HorizontalThumbHeight, HorizontalThumbWidth, VerticalThumbWidth,
VerticalThumbHeight
- change height/width of the thumb
Merge pull request #791 from MahApps/punker76-minor-slider-changes

minor changes to slider style
demo for IsIndeterminate runtime change
enable runtime change of IsIndeterminate
Merge pull request #792 from MahApps/punker76-metroprogressbar-change-IsIndeterminate-at-runtime

enable runtime changing of metroprogressbar indeterminate property