RFC: replace "icon spacing" settings in Icon Task Manager and System Tray with a panel-level "density" setting

The main problem with the current setting: in the default panel (as in the screenshot in https://kde.org), the user would perceive Kickoff and Icon Task Manager as a single row of icons, instead of two applets. But if the user change the icon spacing setting, the spacing between the Kickoff icon and the rest of icons becomes uneven.

So I propose lifting the setting up into the panel config dialog. All applets in the panel should set their margin according to the panel setting (supposedly implemented in some common code). And containers like Task Manager and System Tray should also set their internal margins.

Moreover, when the panel is full, it could reduce the density (just like Task Manager currently does), so all applets shrink simultaneously. Thus, while the user-facing "density" setting probably has only three options, e.g. Compact, Normal, Spacious, the actual setting passed to applets should be in px.