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
Lower required version of GTK+
view: Replace Gtk.Stack with Gd.Stack
window: Replace Gtk.Stack with Gd.Stack
toolbar: Replace Gtk.StackSwitcher to Gd.StackSwitcher
Headerbar.ui: Replace Gtk.HeaderBar with Gd.HeaderBar
toolbar: Don't set custom title on Toolbar init

This makes sure that the label created by the header is realized on init.
The setting of the state of the toolbar to 0 will do this anyway.
window: Pack header bar if GTK+ version is older than 3.9.0

gtk_window_set_titlebar was introduced in 3.9.0.
toolbar: Destroy the window on close if GTK+ version is older than 3.9.8

gtk_window_close was introduced in 3.9.8.
window: Hide titlebar when maximized if GTK+ version is older than 3.9.0

We can't set a custom titlebar before 3.9.0, so hide the titlebar from the
window manager when maximized.