Selection-after-split behavior (gtk2 part)

Selection-after-split behavior (gtk2 part)

*  When splitting in MouseObject, entered_region should get priority over selected regions.
    This fixes the unexpected case where you try to split an unselected a region, but
       a)  nothing happens  OR
       b)  some other region (maybe off-screen) is split

*  Range mode now has its own option for splits, which can be:
     Clear:  the selection is cleared.
     Preserve:  the selection is left as-is. (default)
     Force:  all the regions that resulted from the split are selected (forcing a tool change).

*  Un-hid the additional config options to select only the regions BEFORE or AFTER a split.

*  Note:  splits made with Cut Tool should be unaffected by these changes.