plugins: Don't unload -> avoid dangling resources
plugins: Don't unload -> avoid dangling resources

The Qt "do not allow unload" patch
(https://codereview.qt-project.org/#/c/140750/) was postponed to Qt5.7

Discussion and agreement on not unloading libraries is here ->
http://lists.qt-project.org/pipermail/development/2015-November/023681.html
So plugins will not be unloadable in future Qt apps.

This solves e.g. mount plugin SEGFAULT described
https://github.com/lxde/lxqt/issues/1013#issuecomment-209793407.

cferr/lxqt-panel,lxde/lxqt-panel,lxde/lxqt-panel,Ilya87/lxqt-panel,stefonarch/lxqt-panel,Ilya87/lxqt-panel,pmattern/lxqt-panel,pmattern/lxqt-panel,cferr/lxqt-panel,ThomasVie/lxqt-panel-de,stefonarch/lxqt-panel,ThomasVie/lxqt-panel-de,stefonarch/lxqt-panel