applets/kicker: fix app icon loading logic to better handle relative paths

ba44b69a added logic to handle apps that
use an absolute path in their .desktop file to define their icon, which
works. However in the process it introduced a subtle bug: if the icon is
not an absolute path and it's just a normal icon name, when
QFileInfo::exists() checks for the existence of that string, it will
treat it as a relative file path and therefore look for it in the
current working directory, which is typically the user's homedir. If it
finds something, it will go down the wrong code path and end up
returning a blank QIcon. This can be verified by adding a folder with
the name of an app icon into ~ and restarting plasmashell; that app in
Kickoff will have a blank icon.

To fix this, the icon loading code now first checks whether the icon
returned by m_service->icon() is actually an absolute path. If not, it
skips the logic to look for it on disk and goes straight to the
codepath that looks for an icon with that name in the icon theme.

To minimize disk reads, it checks for absolute-file-path-ness by
inspecting the string returned by m_service->icon() rather than using
QFileInfo::isAbsolute(), because this is a hot code path and most icons
will not have relative paths, so checking the disk for every one of
them would be a waste of resources.

BUG: 457965
FIXED-IN: 5.24.7


(cherry picked from commit 57d55e38)