applets/taskmanager: show context menu on press-and-hold
requested to merge ngraham/plasma-desktop:press-and-hold-for-taskmanager-task-context-menu into master
This makes the applet more touch-friendly. Most applets are designed in such a manner that they don't have any features that can only be accessed from a context menu, but the Task Manager is an exception, and it's not at all simple to envision how we could move all the items in the context menu to a visible UI, or if it would even be feasible or desirable to do so at all.
Thus, let's just show the normal context menu with a press-and-hold.
Code is lifted from Kickoff, including the TODO.
BUG: 445901 FIXED-IN: 5.25