wcm: support metadata files installed by wayfire-plugin

[dkondor]

Fixes #123

This is the simple way, hardcoding the same path logic that Wayfire uses. As discussed in #123, an alternative would be for Wayfire to export the (relative) paths via pkg-config or similar mechanism. However, I'm now in favor of this simpler option, since:

• I expect that the paths will likely not change often
• Any such change would require rebuilding wcm anyway, while the update needed here would be trivial

A further alternative would be for Wayfire to instead always provide the WAYFIRE_PLUGIN_XML_PATH instead -- while maybe a bit more complex, in that case, no change to wcm would be needed.

[soreau]

This is par for the course, so far as wcm is concerned. LGTM, I will assume you've tested this.

[soreau]

Thanks!