Add fallbacks for icon names only the Mint themes provide

[Fantu]

A few of the icon names Cinnamon asks for are not part of the freedesktop Icon Naming Specification and are provided by Mint-X and Mint-Y only. On a distribution that ships a different default icon theme they resolve to nothing and the GTK "missing image" icon is drawn instead.

This was reported on Debian as #1147525: the Recent Files category of the menu applet is missing its icon with every icon theme available there (gnome, mate, Adwaita, ContrastHigh).

Rather than replacing the names — which would change how these icons look on Mint — this asks for a GIcon with a list of names, keeping the current name first and adding a fallback that exists everywhere:

first choice (unchanged)	fallback	where
folder-recent	document-open-recent	"Recent Files" category of the menu applet
gnome-panel-launcher	cinnamon-panel-launcher	launchers with no icon of their own
application-default-icon	application-x-executable	windows with no application icon (window list, grouped window list, alt-tab, workspace overview)

Both fallbacks of the first kind are in the naming spec; cinnamon-panel-launcher is installed by Cinnamon itself from data/icons, and is already the icon panel-launchers@cinnamon.org declares for itself.

Verified with GtkIconTheme.lookup_by_gicon:

• Mint-X → Mint-X/places/48/folder-recent.svg, Mint-X/apps/48/gnome-panel-launcher.png, Mint-X/apps/48/application-default-icon.png — all unchanged.
• Mint-Y → Mint-Y/places/48/folder-recent.png, Mint-Y/apps/48/gnome-panel-launcher.png — unchanged; application-x-executable for the third one, which Mint-Y does not have under the old name either (so this is a missing icon on Mint too, now fixed).
• mate-icon-theme (the Debian default) → document-open-recent, cinnamon-panel-launcher, application-x-executable instead of nothing.

Two notes:

• The symbolic category icons of the menu applet are unaffected: get_names()[0] is still folder-recent, so the "xsi-" + icon mapping keeps resolving to xsi-folder-recent-symbolic.
• The "icon" fields of the applet/desklet metadata are deliberately left alone — ExtensionCore already checks has_icon() and falls back to cs-applets/cs-desklets, so those never show a broken icon.

[JosephMcc]

The proper fix is for Cinnamon to ship any non-standard icon it uses as part of it's packaging or include it in one of it's dependencies, like xapp.

[Fantu]

That is already what one of the three cases does: the panel launcher fallback here is
cinnamon-panel-launcher, which Cinnamon ships from data/icons and which
panel-launchers@cinnamon.org already declares as its own icon. No new artwork needed
there.

For the other two I would argue the name list is the better fix, because of how GTK
resolves a GIcon: theme by theme, not name by name. It tries every name in the current
theme (and its inherited ones) before falling through to hicolor. With a folder-recent
shipped in hicolor and mate as the icon theme:

Gio.ThemedIcon.new_from_names(["folder-recent", "document-open-recent"])
  → /usr/share/icons/mate/48x48/actions/document-open-recent.png

So a shipped icon always beats a theme that has the specification name, and the user ends
up with our icon sitting among icons drawn in a different style. With the name list the
theme keeps the first word — Mint-X/Mint-Y through folder-recent, other themes through
document-open-recent — and the result stays visually consistent either way. Both
folder-recent and application-default-icon do have an equivalent in the naming
specification, unlike cs-* or cinnamon-panel-launcher, which are genuinely Cinnamon's
own.