[FFe] GDM background configurable with gsettings

[Jean-Baptiste Lallement]

[Description]
Customization of the greeter is a requirement from Corporate customers.

Currently the background (plain color or image) of the greeter is hard coded in the style embedded in the theme. Customizing this background requires to extract the style sheet from gnome-shell gresource file, modify it manually, recompile and deploy the new gresource, and add an alternative to it.

This feature adds 4 gsettings keys to make the background of the greeter configurable via gsettings. The default value defined the in the theme is used for keys that are not set.

[Rationale]
Corporate customers use LTS releases and ultimately this will be released for 20.04.
This changeset needs to land in hirsute before it can be SRUd into prior Ubuntu LTS releases. So we would like to submit this FFe to get it into Hirsute first.

[Risks]
The changeset is minimal, isolated and can be easily reverted.
In case of error in the code, GDM will fail to load and it will be immediately visible.

[Build]
Builds and tested successfully locally.
Besides the updated JS is not compiled as part of build.

[Upstream Issue]
https://gitlab.gnome.org/GNOME/gdm/-/issues/684

[Marco Trevisan (Treviño) (3v1n0)]

Not sure if the gsettings schema change should be part of gdm though, as if this will ever be upstreamed that's where it will belong.

Indeed, it will makes the FFe a bit more complex and cause more changes though, but we'd get there anyways when (if) we'll have to migrate to gnome keys.

[Jean-Baptiste Lallement (jibel)]

Having the schema in GDM was our initial proposal but you told us that it would be better to do it the way we just proposed and do it in an independent schema as long as it is not yet accepted upsteam. In both cases there will be a migration *if* something similar is ever implemented in gnome-shell.

[Łukasz Zemczak (sil2100)]

At first reading this FFe made me a bit weary, since the operation of changing the color in the style sheet is not as easy as just changing a value and done. But then seeing the change itself, and the fact that this is isolated to only one component, I think it's still 'okay' to get this in. Please make sure it migrates before the Beta Freeze. FFe approved!

[Łukasz Zemczak (sil2100)]

(but please do forward this to appropriate places, since I see the debdiff had Forwarded: TODO still ;) )

[Launchpad Janitor (janitor)]

This bug was fixed in the package gnome-shell - 3.38.3-3ubuntu2

---------------
gnome-shell (3.38.3-3ubuntu2) hirsute; urgency=medium

  [ Didier Roche ]
  [ Jean-Baptiste Lallement ]
  * debian/patches/ubuntu/configure_login_screen.patch:
    Make GDM background configurable with gsettings (LP: #1918613)

 -- Didier Roche <email address hidden> Thu, 25 Mar 2021 10:44:17 +0100

[Daniel van Vugt (vanvugt)]

Regressed in 44? See bug 2018672