[applets/taskmanager] Make minimizing the active task on click optional

When this new option is flipped off, clicking on the active task will do
nothing instead of minimizing it. This is the behavior of the macOS
Dock, and adding this option finally makes it possible to perfectly
replicate its behavior in Plasma, in conjunction with the new default
"cycle through grouped tasks on click" option. Being able to do so is
useful for theme creators making macOS Global Themes of which there are
tons and tons, or people switching from macOS who want to generally use
a Plasma look and feel but want to replicate the behavior of the Dock
that they became accustomed to.

BUG: 405495
FIXED-IN: 5.20