TaskManager: add icon to cache after painfully querying it over XCB/NETWM.

Summary:
Hitting again a bug where switching desktops shows a delay of several seconds
before the taskbar updates, I noticed that this code path was called 81
times in 7 seconds (!) for kmail (maybe because I start it from cmdline
rather than via a .desktop file).

After the 4 slow calls to KWindowSystem::icon it seems definitely worth
it to cache the result.

Overall the main performance bug is still there though, switching
desktops is still slow, but now all backtraces show QML triggered
by model/view dataChanged signals, no idea yet, other than hoping
for the pending QSFPM patches to help with this (no need to re-sort
when the dataChanged roles are unrelated to the roles used for sorting).

Reviewers: hein

Reviewed By: hein

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D4600