Add theme for gdm login page and change structure of repo

Created by: meshya

Reason of change structure:

Testing Package was not possible 'cause source had set to git@github:parchlinux/gnome-parch

changes:

1. add settings file /etc/dconf/db/gdm.d/95-parch-gdm-config
2. add logo file /etc/gdm/gdm-login-logo which used in 95-parch-gdm-config
3. add gresource file /usr/share/gnome-shell/gnome-shell-theme.gresource which is parch-gnome-shell-theme.gresource in the package and will replace with /usr/share/gnome-shell/gnome-shell-theme.gresource in post_install