Clean gsetting schemas installation

[gsemet]

Glib/gsettings schemas is actually installed twice:

• once in /usr/share/glib-2.0/schemas to dconf-editor is happy
• once in /usr/local/lib/python3.5/dist-packages/guake/data/ and used by guake

need to change so only one is used. Need to support development mode where is is not yet installed.
My guess is that:

• development, manual install through sudo make install and pypi install would use the Guake's data directory, so there is no modification of the system. dconf-editor might not see this schema.
• package installation (apt install guake) would use glib-2.0 schemas directory, allowing using to use dconf-editor.

[aichingm]

Don't forget /usr/lib/python3.{5,6}/site-packages/guake/data/ for none debian users 👍 (#1122)

[jtojnar]

Why are you installing the data to pythondir? If you used $(prefix)/share, GLib would pick up the schemas automatically (it checks XDG_DATA_DIRS).

For testing without installation, you can do something similar to what, for example, GNOME Music does:

1. Compile the schemas to the build dir: https://gitlab.gnome.org/GNOME/gnome-music/blob/44d21816a0b87f84f631faf82d3012a44355238c/meson_post_conf.py
2. Create a config template, that will extend the XDG_DATA_DIRS variable with the builddir: https://gitlab.gnome.org/GNOME/gnome-music/blob/44d21816a0b87f84f631faf82d3012a44355238c/gnome-music.in#L43
3. Configure the file: https://gitlab.gnome.org/GNOME/gnome-music/blob/44d21816a0b87f84f631faf82d3012a44355238c/meson.build#L73-89

[gsemet]

OK, i'll see that. I actually always found these extremely complex, without even finding a clean and simple "good practices guide".

[gsemet]

Relates to #1236

[gsemet]

fixed on HEAD