No "render between loop markers" option in CLI

We currently support exporting as a loop via --loop switch. However, there's no CLI counterpart of "render between loop markers" option.
The functionality was added by @curlymorphic via #1482, but it's available only in GUI. Adding the CLI option itself would be easy, but it needs some changes in the codebase.
I've tried simply adding the option, but it resulted in crashes. It's because the SongEditor constructor initializes Song::m_playPos[Song::Mode_PlaySong] via creating a TimeLineWidget instance. TimeLineWidget stores loop settings and it doesn't even get loaded in CLI mode.
https://github.com/LMMS/lmms/blob/f37ca49e6d52886d40858e23acd1d3ed32f97a71/src/core/Song.cpp#L1106-L1107
https://github.com/LMMS/lmms/blob/f37ca49e6d52886d40858e23acd1d3ed32f97a71/src/core/Song.cpp#L1124-L1127
GUI-independent parts of TimeLineWidget should be pulled out in order to fix the problem. I expect it will require some refactorings as well.

Start with the referenced Song.cpp sections around lines 1106-1107 and 1124-1127, then inspect TimeLineWidget and the existing --loop CLI handling. Separate the GUI-independent loop settings needed by CLI mode and verify that rendering between loop markers works without the previous crash.