Improve OSD usability

[romangg]

Current state

The OSD is activated when:

• a second output is connected and no configuration file exists for the two outputs.
• the hardware key on laptop keyboards is pressed.

The OSD provides the following options:

• Switch to external screen
• Switch to laptop screen
• Unify outputs
• Extend to left
• Extend to right
• Leave unchanged

Problems

• The OSD also should show for workstations. The naming "Switch to laptop screen" etc. is false then.
• Currently the "Switch to laptop screen" option is disabled, but it should show when invoking the OSD via hw-key. It should not show when a new external display is connected. It doesn't make sense to connect a new display and then disable it directly - or it could if the compositor has it disabled by default. Use case: presentation on beamer, setup everything first and only then enable it.
• Only works for two displays. But at least extending left or right could be done for more displays too. Also replication could if the other displays are already in replica-mode.