libtaskmanager: improve efficiency when window icon frequently changes

Reason for the change

Don't wipe the whole appData cache entry when only the icon changes because constructing the cache requires querying all applications on the local disk.

Add NET::WMIcon to cachable properties as it's usually expensive to update icons.

Test plan

Pass xwindowtaskmanagertest

Modify the interval in libtaskmanager/autotests/data/windows/bug478831.py to 1 and run the script

Screenshots or screen recordings

Bugs fixed

BUG: 487390 FIXED-IN: 6.1.2