DRM backend without any outputs connected

Currently, if there is no any physical output connected, kwin will wait until one is connected and proceed with startup; or if the last output is disconnected, it won't notify the rest of kwin about it, which is a wild territory as it leads to inconsistency between Screens and platform outputs.

KWin should gracefully handle the case where there are no outputs. We may also need to consider the possibility of creating a virtual output if there are no outputs, however kwin still needs to handle the case where there are no outputs.

Sub-tasks:

• port Workspace away from Screens, port most of int-based screen api in Toplevel to AbstractOutput
• make Workspace and AbstractClient handle gracefully absence of any outputs connected
• introduce DrmPlaceholderOutput or something?