Fix crash in core plugins

Sorry if I haven't described this bug before, but I discussed about it last night on IRC channel together to @timlinux and @jef-n who has already fixed the coordinate capture plugin.

This issue occurs when you disabled a core plugin (spatialquery, georeferencer etc) and subsequently you try to change some setting in option dialog. It is happening because the ```currentThemeChanged``` signal is emitted (I am guessing also other signals), just simply hitting the OK button without changing anything,and it tries to change the current theme  for the plugin action button which is not unloaded properly. I had thought to comment out the line that connects at the ```setCurrentTheme``` slot, since we have only the default theme, but Jürgen has found a more elegant solution.

Also note that I was able to test grass and compass plugin which also are affected as they are using that signal.
Could anybody else fix grass and compass plugin ?

So if this patch is fine for you, please cherry pick it to release-2.0 branch.

Thanks!