Make signal API identify signals by their signal data and not arbitrary strings

[ammen99]

@Javyre thoughts ?

Eventually, this should fix #1307

If anybody is interested in why I want to do this change, some of the rationale is described here: https://github.com/WayfireWM/wayfire/commit/22741481159b28a7c7204c87d3d3530b85902e83

Also, this new implementation can be improved even further to use static storage for different signals (thus eliminating the unordered_map lookup), also we can use a simpler safe_vector_t-like type to make iteration cache-efficient and lower the overhead of signals even further, if need be. Currently, signals are not that critical for performance and I have opted for a simpler (and a bit slower) implementation.