When constructing MetaMonitorsConfig objects, store which type
of switch_config they are for (or UNKNOWN if it is not such
type of config).

Stop unconditionally setting current_switch_config to UNKNOWN when
handling monitors changed events. Instead, set it to the switch_config
type stored in the MonitorsConfig in the codepath that updates logical
state. In addition to being called in the hotplug case along the same
code flow that generates monitors changed events, this is also called
in the coldplug case where a secondary monitor was connected before
mutter was started.

When creating the default linear display config, create it as a
switch_config so that internal state gets updated to represent
linear mode when this config is used.

The previous behaviour of unconditionally resetting current_switch_config
to UNKNOWN was breaking the internal state machine for display config
switching, causing misbehaviour in gnome-shell's switchMonitor UI when
using display switch hotkeys. The lack of internal tracking when the
displays are already in the default "Join Displays" linear mode was
then causing the first display switch hotkey press to do nothing
(it would attempt to select "Join Displays" mode, but that was already
active).

Fixes: #281
!213