Improve api consistency

These changes are designed to improve overall API consistency.  These
changes are mostly just name changes, though there is at least one
change where a function was removed and replaced with better functions
(in https://github.com/jp9000/obs-studio/commit/a79311b3b6a1f3cc8389c00bf3115089ec3ddd5f) or split up in to two functions (https://github.com/jp9000/obs-studio/commit/97004df5112685627ef58ff3f13bd3103df23f3e).

These changes ensure that
- Major API names are not unnecessarily squished
- Major API functions that are used for data retrieval have "get" in the
  name, rather than implying that they return data

Each change is marked in each commit.