Fix crash when icon not found in either theme directory

[Onyx47]

Fixes crashing with some applications which only provide icons in /usr/share/icons. Tested with qbittorrent and keepassxc which crash the current version.

Also doesn't crash on applications which only provide the icon as a pixmap (tested with discord) - it will now show the default "icon not found" pixmap instead.

Unrelated but leaving a note here: qbittorrent might show two entries but this is actually because it does spawn two notifier items, so some extra filtering might be required for some applications.

EDIT: Added pixmap loading from DBus. There is some code duplication since I didn't want to remove existing code as of right now.

[NicTanghe]

I get the following error when i try to check it out.

❯ gh pr checkout 2
fatal: couldn't find remote ref refs/heads/add-example-config

MAybe i have a wrong order of operations ?

This is what i did.

❯ gh pr checkout 743
remote: Enumerating objects: 131, done.
remote: Counting objects: 100% (131/131), done.
remote: Compressing objects: 100% (55/55), done.
remote: Total 131 (delta 76), reused 113 (delta 76), pack-reused 0
Receiving objects: 100% (131/131), 55.86 KiB | 3.72 MiB/s, done.
Resolving deltas: 100% (76/76), completed with 6 local objects.
From https://github.com/elkowar/eww
 * [new ref]         refs/pull/743/head -> tray-3
Switched to branch 'tray-3'

eww on  tray-3 via  via 🦀 v1.70.0
❯ gh pr checkout 2
fatal: couldn't find remote ref refs/heads/add-example-config

[eclairevoyant]

@NicTanghe You're on the wrong repo, this is ralismark/eww not elkowar/eww

gh pr checkout 2 on your local clone will checkout elkowar#2, not this PR.

[ralismark]

I've now cherry-picked most of the contents of this PR (which now has a bunch of my commits because I rebased... oops). Thanks for this!