Added Slider visible component. A Slider is like a ProgressBar that adds
a draggable thumb. You can touch the thumb and drag left or right to set the
current slider position. This triggers PositionChanged, reporting current
position of the slider thumb.
The slider has min, max and initialPosition properties.
The reported thumb position can be used to dynamically change another
component property; such as TextBox font size or Ball radius.

Change-Id: I30d6714c3b6307776c6b211ae8797cd0c81edec8
Merge branch 'master' of github.com:mit-cml/appinventor-sources into errorslist