Add device icons to KDE Connect context menu

Firefox supports setting icons on menus [1]. Chrome refuses to create a menu with unknown keys,
so this is guarded by IS_FIREFOX.
This uses the appropriate Breeze icons, except "computer-tablet" which I edited to have larger
padding to somewhat fit better with the rest of the icons.

[1] https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/menus#Icons

Differential Revision: https://phabricator.kde.org/D28837