add configurable configuration backend

[ammen99]

This PR adds the ability to change the configuration backend used by Wayfire.

The default configuration backend simply uses wf-config APIs, however, another backend could load
the settings directly from gsettings or whatever other format is desired.

Fixes #725

There are also changes to the output and input-device configuration syntax.
They now use the newly added object type
(and the newly added output types WayfireWM/wf-config#37),
which can then be used to automatically type-check all output options. This should also
make it easier to write an output configuration section in WCM because of the easier parsing.

The only change required in the .ini file is to append output: or input-device: in front of the corresponding sections.
For example, [eDP-1] becomes [output:eDP-1].

[ammen99]

Also if someone tries that, keep in mind I have not had much time to test, so there might be some instability (although my tests so far indicate it works just fine).

[damianatorrpm]

For example, [eDP-1] becomes [output:eDP-1].
With -B /usr/lib64/wayfire/libdefault-config-backend.so
This doesn't seem to be enough. The output configuration is not used.
Note: I have both [DP-1] and [output:DP-1] in the config file, same for DP-2, HDMI-A-1 and HDMI-A-2.

[ammen99]

For example, [eDP-1] becomes [output:eDP-1].
With -B /usr/lib64/wayfire/libdefault-config-backend.so
This doesn't seem to be enough. The output configuration is not used.
Note: I have both [DP-1] and [output:DP-1] in the config file, same for DP-2, HDMI-A-1 and HDMI-A-2.

As discussed on IRC, this is because of an invalid value for the output mode. The refresh rate is always an integer, either in Hz or mHz.