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(cherry picked from commit 0192ff6b)

(cherry picked from commit a647d65c)

Add external schemas, and build a reference gschema.compiled. At
startup, verify that the installed compiled schemas conform to that
reference schema; if not, use the reference schema as schema source
instead.

This fixes the problem where, on upgrade, or through an otherwise failed
recompile of the gschemas.compiled cache, where the upgrade to
gnome-terminal added ettings keys, the server will abort when trying to get
that new key.

E.g. https://bugzilla.redhat.com/show_bug.cgi?id=1961775 which is just
the latest instance of this problem occurring.

Specify the GSettingsSchemaSource to use when constructing a GSettings
object. Due to glib#1884 this
neccessitates not using g_settings_get_child() to construct a child
setting, but instead to construct the child GSettings directly.

In preparation for the next commit.

(cherry picked from commit cbeba197)

Fixes: #361

(cherry picked from commit 67a00883)

(cherry picked from commit c99ffc30)

(cherry picked from commit da268112)

(cherry picked from commit ae58daae)

(cherry picked from commit 3b24c55f)

(cherry picked from commit 22a40309)

(cherry picked from commit 33557f10)

(cherry picked from commit ed959c90)

(cherry picked from commit c0353e8f)

Update the icon in the docs to use the same SVG icon as the desktop
file, and add a trail link.

Fixes: #335

(cherry picked from commit 3128c681)

(cherry picked from commit aac78a71)

(cherry picked from commit c353f952)

(cherry picked from commit c430bbe2)

(cherry picked from commit f84e0f13)

(cherry picked from commit 9b2ff07f)

(cherry picked from commit 29310ead)