applets/taskmanager: show context menu on press-and-hold

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