Add a helper to access config.h from public API headers

[ammen99]

Currently, to include config.h in the public API headers, we need a check whether we are in Wayfire's own repo (where the path to the file is simply config.h), or whether we are being used from an external plugin, where we need to use wayfire/config.h instead. Since this is duplicated in multiple headers, it would probably make sense to add a common header file which can work in both cases, and contains the check.