taskbar: Fix incorrect move between multiple instances
taskbar: Fix incorrect move between multiple instances

With old implementation if button is dragged between multiple instances
of taskbar plugin could lead to invalid layout move -> application
crash (SEGFAULT).

stefonarch/lxqt-panel,pmattern/lxqt-panel,DrTobe/lxqt-panel,jubalh/lxqt-panel,pmattern/lxqt-panel,rbazaud/lxqt-panel,jeremywh7/lxqt-panel,stefonarch/lxqt-panel,stefonarch/lxqt-panel,ThomasVie/lxqt-panel-de,Ilya87/lxqt-panel,cferr/lxqt-panel,DrTobe/lxqt-panel,ThomasVie/lxqt-panel-de,rbazaud/lxqt-panel,jubalh/lxqt-panel,cferr/lxqt-panel,lxde/lxqt-panel,Ilya87/lxqt-panel,lxde/lxqt-panel,jeremywh7/lxqt-panel