wxSplitterWindow mouse capture improvements and cleanup.

wxSplitterWindow mouse capture improvements and cleanup.

- Handle mouse-capture-lost event to abort dragging mode.
- Remember mouse and sash position on buttondown event and use them as
  absolute reference during dragging. Avoid delta values from one mousemove to
  the next as this may introduce a skew during dragging and especially when
  coordinate clipping occurs.
- Clear the requested sash position when dragging in live mode.
- Draw the tracker at correct coordinates - taking into account the width of
  the pen used to draw the tracker.
- The old code did not clearly distinguish between live vs. tracking mode in
  some places.

Closes #11076.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@61615 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775