Fix a bug in the comparison function that could result in the wrong
index being assigned to a monitor. Change the logic to more directly map
to the description.

macOS and Windows present mirrored monitors as a single virtual monitor,
but X11 exposes this scenario as two distinct monitors with identical
coordinates. This messes up our logic, and is likely confusing for the
user. So instead we'll ignore any monitors that have identical
coordinates to any already seen monitors.

Try to reuse and split things so that it makes things easier for the
translators.

We don't want the log to spam every time this parameter is used so only
complain from the parses when the value is changed.

This class is generic and should not reference a specific instance of
it.

Try to be consistent with existing log messages.

Fix some indentation issues and make sure everything* uses CamelCase.

* MonitorArrangement is left as snake_case in order to pretend it is an
  independent FLTK widget class.

The user might not always want to use all monitors when in fullscreen
mode, but instead only a few. This commit adds support for configuring
selected monitors from command line, in the config file and graphically
in the options menu.

Because it might be hard to guarantee the consistency of monitor
identifiers coming from third-parties (for example FLTK), it has been
decided to use our own numerical identifier. This identifier is based on
the monitor's positions. The mapping between this identifier and the
indices used by FLTK is done by MonitorIndicesParameter.