applets/taskmanager: show text more often for traditional Task Manager

The minimum area for text was using a PlasmaCore unit that got ported to Kirigami.Units.iconSizes.sizeForLabels for Plasma 6. This has different semantics, resulting in the minimum size to show text increasing hugely. As a result many people stopped seeing text in their task managers with medium-thickness vertical panels crowded horizontal panels.

The more semantically correct iunit to port to is GridUnit. It's not perfect though since it's the font height, not width. But it's the best we've got.

In my testing, 1.5 of them works perfectly for both vertical and horizontal panels to only show text when there's at least one character that won't be elided. This isn't a lot of text, but pepole use a traditional Task Manager because they want to see text, so it at least better matches their expectation than the status quo. BUG: 483070 FIXED-IN: 6.0.4

Some representative befores-and-after:

Before, very full 42px horizontal panel	After, very full 42px horizontal panel

Before, vertical 86px panel	After, vertical 86px panel