plugins/overview: Desktop Bar height configuration

The Desktop Bar thumbnail previews are quite small on high-resolution monitors. This requires more cursor precision when flicking windows to a target desktop. A small count of virtual desktops results in wasted horizontal space as well.

This occurs at base Global scaling (100%). The Desktop Bar currently does not respect global scaling if set (BUG 497149), but this is unrelated.

Taking inspiration (and terminology) from the recent sizing improvement to applets/taskmanager, I'm proposing a similar feature for the Desktop Bar.

Check out how much easier it is to see what's going on in each desktop thumbnail. Larger targets to flick to, and it better uses screen space:

This MR adds integer value desktopBarHeight and multiplies the Desktop Bar's default height of Kirigami.Units.gridUnit * 5 by this multiplier. This also allows for the bar to be disabled with the default index of 0. The rest of the Overview effect will adjust to fit each size without any other changes.

The current status quo is the Narrow option in this MR. The new Medium and Wide sizes are a pleasure to use, while Narrow is provided as a fall-back option for existing users. I enthusiastically insist on providing user configuration for this setting, just as the example Task Manager applet did.