switch for only fixing app / base icons

[andia89]

We should implement a switch (could be a not public one) , that allows changing only the app icons / base icons. This would allow to restore old settings when upgrading circle and or base individually (after adding a simple command to the POSTINST script of course)

[bilelmoussaoui]

I think we can use gsettings for that and also fix #135. This way we will also have to introduce an installation script (makefile)