monitor-manager: Let the MetaMonitor derive the whole layout

Instead of letting MetaMonitor derive the logical monitor size, then
using the main monitor for the position, just let MetaMonitor derive
the whole layout including the position. This means it can deal with
tiled monitors better, for example when the main output (the output
always active when the monitor is active) is not the origin output (the
output with tile position (0, 0)).

https://bugzilla.gnome.org/show_bug.cgi?id=781723