Bug 1082486 Part 4 - Check whether nsCaret shows in the scroll frame. r=roc

This patch check whether nsCaret shows in scroll frame. If yes, touch
caret will show, and its position will be clamp to scroll frame as
before. Otherwise, we hide it since the nsCaret will not be seen in this
case.
Bug 1082486 Part 5 - Hide touch caret when PresShell is suppressing painting. r=roc

When loading an html, painting is suppressed in PresShell. Therefore the
rect of nsCaret or the rect of nearest ancestor scroll frames will not
be correct. Touch caret will hide incorrectly because it cannot get the
necessary rect to calculate its position.

I added a condition in IsDisplayable() to skip when painting is
suppressed. Touch caret should sync its visibility with nsCaret again
when painting is unsuppressed, and those rects needed by touch caret
should be ready then.