No "render between loop markers" option in CLI

[PhysSong]

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.

[PhysSong]

Reopening since #7004 itself doesn't add the required feature.