Add searching for plugins instead of a hardcoded list

Currently playing around with an out-of-tree plugin, so the hardcoded plugin list got quite annoying.

Maybe not the best implementation, like, it could be integrated into libobs to just a single function call, but for the meantime this is a quick and easy solution.
It fails to load plugins for me because obs_load_module expects the name to not be prefixed with "lib".
Maybe do something like:
```C++
QString baseName = finfo.baseName();
if (baseName.startsWith("lib"))
	baseName.remove(0, 3);
obs_load_module(baseName.toUtf8().constData());
```