applets/taskmanager: fix size of textual list popup

The contextual list popup for grouped tasks is restricted to a maximum size of (screen width / 3) × (screen height / 2). For both, the denominator is additionaly scaled by the devicePixelRatio. This scaling does not really make sense, as the values are already in logical pixels, so the only thing it does is that users with a scaling factor over 100% get a smaller popup (in terms of actual area) that has space for fewer items before scrolling becomes necessary. This is particularly pronounced on Wayland, where the devicePixelRatio seems to be rounded up to the next integer currently, and even small increases in scaling lead to a huge reduction in dialog capacity.

This change removes the additonal scaling; the maximum dialog size is now always a third of the actual screen width and half the actual screen height.

Inappropriate size in action (Wayland, 125% scaling):

Correct size in action (Wayland, 125% scaling):

NB: It seems to me that even 50% height is rather stingy; textual list seems mostly for users that tend to have lots of windows open, who would likely prefer a longer list over having to scroll - 60% or even 75% would likely be ok. The screen width constraint seems fine; if it's too long the text becomes hard to read.