(cherry picked from commit 7ec0471)
This isn't a real cherry pick, as I've only applied the changes
affecting Signal (these changes are required to cherry-pick further
commits) and customized the subject to avoid confusion.

(cherry picked from commit e1b501a)

(cherry picked from commit 1ac4be3)

(cherry picked from commit 507d24a)

If executed in a pure environment (nix-shell --pure) or depending on the
`gtk3` version of the system Signal-Desktop was e.g. crashing when
clicking on a PDF attachment (instead of showing the dialog to save a
file).
Using wrapGAppsHook and setting XDG_DATA_DIRS to the correct version
fixes this bug.

The error message was the following:
```
(signal-desktop:30756): Gtk-WARNING **: 14:04:49.073: Could not find the icon 'user-home-symbolic-ltr'. The 'hicolor' theme
was not found either, perhaps you need to install it.
You can get a copy from:
        http://icon-theme.freedesktop.org/releases

(signal-desktop:30756): GLib-GIO-ERROR **: 14:04:49.134: No GSettings schemas are installed on the system
Trace/breakpoint trap
```

(cherry picked from commit 5d79535)

Removed some redundancy (src check via meta.platforms) and made some
changes according to our style-guide.
I've changed meta.description and added meta.longDescription.

(cherry picked from commit ab593d4)

(cherry picked from commit 54ea1cd)

(cherry picked from commit db0da28)

(cherry picked from commit c4d94d6)

(cherry picked from commit c4fca64)