gtk3: single click with selection set doesn't unselect

this is a safe backportable fix, the unconditional

nFlags |= SelectionEngineFlags::CMDEVT

looks the real dubious thing to me, but only the gtk3 backend has swipe support
so just returning immediately on a swipe definitely ok

Change-Id: Iade26d0d9260ce76dfb2766ac9e5ca9fa9275bf4
Reviewed-on: https://gerrit.libreoffice.org/48527Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

vcl/source/window/seleng.cxx

@@ -342,7 +342,7 @@ void SelectionEngine::Reset()
 void SelectionEngine::Command( const CommandEvent& rCEvt )
 {
     // Timer aWTimer is active during enlarging a selection
-    if ( !pFunctionSet || !pWin || aWTimer.IsActive() )
+    if ( !pFunctionSet || !pWin || aWTimer.IsActive() || rCEvt.GetCommand() == CommandEventId::Swipe )
         return;
     aWTimer.Stop();
     nFlags |= SelectionEngineFlags::CMDEVT;