Watch for icons directory changes from KDED

Upon KIconTheme construction folders are checked to exist and only existing ones are used later for lookup of icons. Now consider that a new program is installed and it installs its icon into a directory inside XDG_DATA_DIRS that before did either not contain an 'icons' directory at all or there was no folder of the specific theme at this specific location. A typically long running program that wants to display such icons like an application launcher, taskmanager, system monitor or window manager won't be able to find these icons. KDED can watch for 'icons' directories or new theme directories (subfolders of 'icons' and notify running applications. KDED seems the correct place to watch for such events, it alreay does so for a bunch of subfolders of XDG_DATA_DIRS. BUG:455702 FIXED-IN:5.96