Unhighlight selection when selection is owner by other window

Unhighlight selection when selection is owner by other window

st marks the active selection using reverse colors in the box selection, but
once that another window becomes owner of the selection, it is very
confusing that st keeps highlight the old selection. Usually terminal
emulators remove the highlight when it is not valid anymore.

X sends a SelectionClear event in this situation, so we only have to add a
callback which unhighlight the selectin box.
---
 st.c |    9 +++++++++
 1 file changed, 9 insertions(+)