Plugincreator

Bring plugin creator load and save dialogs into line with corresponding score dialogs.

This means that 

1) the plugin creator will only save files named with a certain number of filename extensions, in this case only ".qml"
2) the plugin file dialog shows the name of the file, not the full path, in the title bar
I'm ok with removing errors but here we would be connecting a signal to a non existing slot. What do we really want to do when the engine quits ?