If there wasn't any scene change yet GetCurrentScene() would always
return nullptr and break various scene checks.
For example, this could happen when startup up a fresh OBS install for
the first time.
The "core" macro conditions and actions have been extracted out to the
"base" plugin.
The library now mostly contains functionality which is required across
all plugins and (e.g. definitions for macro segments).
The goal is to reduce the complexity and cross-dependencies and group
the source files in a better way.
This should relsove the "library limit of 65535 objects exceeded" build
issue occuring in some Windows build environments.
