split off gps and cava modules into run-time loaded shared objects

[qbe]

rebased my work from #4661 , a fix for #4276 .

@Alexays , I would appreciate your input on this

[qbe]

cava-glsl seems to work, whether to load the glsl code (and therefore its dependencies) when the waybar-module-cava.so is loaded is currently a compile-time option (e.g. HAVA_LIBCAVAGLSL controls whether the waybar-module-cava.so has a load-time dependency on epoxy).

This could be reworked into another use of dlfcn.h -based dynamic loading, to enable use of cava "raw" mode on systems that don't have epoxy installed with the same package that enables cava glsl usage.

@Alexays I know that this branch already contains lots of changes, but I am willing to keep it synchronized to let you merge smaller or more self-contained changes quicker. I would nevertheless be interested in your opinion on the whole runtime-loading-for-easier-packaging thing, to see if I have a chance of ever getting this (or something like it) merged.

[qbe]

closed in favor of rebase ( #4735 )