Fix cursor navigation in math.

Fix cursor navigation in math.

* metricsinfo.h:
  - ScreenUpdateStrategy: new enum describing the screen update strategy (replace singlepar boolean).

* UpdateFlags.h:
  new Update::Decoration flag.

* BufferView.C
  - update(): takes Decoration flag into account (CoordCache is not cleared out). Fills in metrics_info_.update_strategy

* mathed/InsetMathNest.C: adjust the cursor flags to new strategy.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@16774 a592a061-630c-0410-9148-cb99ea01b6c8