Improve hit detection in wxMSW wxTreeCtrl with wxTR_FULL_ROW_HIGHLIGHT style.

Improve hit detection in wxMSW wxTreeCtrl with wxTR_FULL_ROW_HIGHLIGHT style.

When wxTR_FULL_ROW_HIGHLIGHT is used, the item visually takes up the entire
breadth of the window so clicking both to the left or to the right of the item
should have the same effect as clicking on it.

So add a MSWIsOnItem() helper which tests for whether a point is above the
item correctly and use it in order to:

1. In multi selection mode, allow clicking anywhere to select the item(s)
   when Ctrl or Shift is pressed (Closes #11598).
2. Generate activation event when clicking to the right of the item too
   (Closes #11602).
3. Detect item bounds correctly in WM_LBUTTONUP handler (although it's
   not really clear what does this code do and hence what problem does this
   fix...).

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63089 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775