Various palette improvements

Various palette improvements

1. Look up the theme background and foreground colors using theme
   resources, if possible.  These theme resources are then used to set
   the foreground and background colors on the labels used as a
   fallback for non-previewable widgets.  In other words, when you're
   looking at a dark theme, the layouts such as LinearLayout is now
   shown using a white label on a dark background, making the palette
   view more consistent visually.

2. Configure scrollbar increments properly such that scrolling the
   views with scrollwheels or mouse gestures works better.

3. Fix a bug in the way preview images were copied out of the rendered
   image; the root coordinates were not taken into account, which
   matters for themes like Theme.Dialog.

Change-Id: I4832166a0560d33fe4e4dd8079c82a180e07e897