kcm: Move device switching to navigation bar
Switching device was previously done by selecting the device in the arrangement view, or by using a combobox in the options panel. This commit moves it to a horizontal tabbar on top, with each display listed — better surfacing disabled displays. In order for this to work, now only the options are in a ScrollView rather than the entire KCM.
In order to have enough room for arranging displays and customising settings for each display, there is now a button to begin editing the arrangement, and to collapse it. This gives more height than there was previously for both tasks, making them easier to work with. With only a single display, both the navigation bar and the arrangement view are hidden.
| Scenario | Before | After | After (editing) |
|---|---|---|---|
| Multi-monitor |  |
 |
 |
| Single display |  |
 |
- |
Please note that the redundant device serial numbers have already been fixed in KWin.
I have plans to adapt !355 (closed) to complement this, such that in the editing mode you will be presented with disabled outputs to drag in/out.