Set GSETTINGS_SCHEMA_DIR if running from source and it is not set

This allows us to run from source without installing the settings file.
Add executable permission to script when created
Remove INSTALL file

Autoconf will create this and shouldn't be tracked.
application: Reshow window if we already have one

The design of gnome-music is to only have one window, so we should just
reshow our main window when activated again.
Merge pull request #87 from kyoushuu/on-source-run

On source run