Each major target of polybar can now be enabled/disabled while
configuring (even polybar itself).
The cmake code specific to each target will only run if the target is
enabled.
This allows us to for example just build the documentation without
having to run all the cmake code related to compilation or having the
polybar dependencies installed (other than sphinx).
atoi, atof and so on have undefined behavior if anything goes wrong. We
now use strto*, but without error checking. In most places overflows and
the like *should* not happen. String to number conversions are only used
when reading data from other applications or from the config, if another
application gives unparsable strings or too large numbers, then most
likely there is something wrong with that application. If the error
comes from the user config, then the user has to live with values
provided by strto* on error (which are very reasonable)
Fixes#1201
