Optimize drawing.

Optimize drawing.

* BufferView:
  - update(): returns a pair of bools in order to inform for singlePar cases.
  - workAreaDispatch(): ditto and simplify the logic by using cursor update flags.

* text.C
  - breakParagraph(): change the cursor update flag as needed.
  - insertChar(): ditto.
  - erase(): ditto
  - redoParagraph(): now returns true if there is a height change.

* text3.C: dispatch():
  - initialize the cursor flag at the beginning
  - LFUN_MOUSE_PRESS: no need to update.
  - LFUN_MOUSE_RELEASE: ditto.
  - add some FIXMEs and comments.

* LyXFunc::dispatch(): simplify the BufferView update.

* insets/insetcollapsable.C: clarify and simplify the logic in doDispatch().

* WorkArea::dispatch(): redraw only if needed.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@15958 a592a061-630c-0410-9148-cb99ea01b6c8