[GTK] Fix/Optimize PanedBackend event handling
[GTK] Fix/Optimize PanedBackend event handling

Fixed to use EnableEvent and DisableEvent, and to use  ApplicationContext.InvokeUserCode within the Handler.
(cherry picked from commit f85e2ad601bfafe7da3c55be6b419ff31b1f4ad8)

Signed-off-by: Vsevolod Kukol <a9b09014047027fe334101e9c575eda8c1abc23b@sevo.org>

hamekoz/xwt,cra0zy/xwt,steffenWi/xwt,antmicro/xwt,lytico/xwt,residuum/xwt,iainx/xwt,mminns/xwt,mminns/xwt,TheBrainTech/xwt,directhex/xwt,akrisiun/xwt,mono/xwt,hwthomas/xwt