taskmanager: Ensure tooltips are really disabled when "Show tooltips" is unchecked

Before frameworks/plasma-framework!358, if enabled is not set (default
true), when the tooltip is disabled in the settings, plasmashell will
still crash at showing window thumbnails, which means the content in
the tooltip is just not visible, but the relevant code is still
executed.

This ensures tooltips are really disabled when "Show tooltips" is not
checked, which is beneficial for people who want to extend the battery
life or avoid the unresponsiveness caused by tooltips.

CCBUG: 444015
CCBUG: 444001