Refactor toggling of command-line logging for import/export

[mihe]

Relates to #90431.

This PR refactors EditorNode::cmdline_export_mode a bit, allowing the new --import command-line option to be able to use it as well.

The changes are as follows:

1. cmdline_export_mode is now called verbose_tasks, since it now needs to encompass importing as well, and I figured maybe something else in the future.
2. This flag is now set from Main::start when importing/exporting, as opposed to from EditorNode::export_preset.
3. The command-line logging that this flag enables is no longer mutually exclusive with the graphical progress dialog, but instead complements it. I couldn't see any reason why this shouldn't be the case. This also means the editor won't freeze up completely for a couple of minutes when doing --export-* without headless.
4. The checks related to EditorResourcePreview have been removed, since they have (from my understanding) largely been made redundant by the headless check in EditorResourcePreview::start() that was introduced by #73838.

[akien-mga]

I saw the rationale for removing the guards for start(), but this one doesn't seem to be a no-op on headless and will still create a MutexLock and possibly trigger more logic.

I haven't evaluated what impact this actually has, maybe it's fine. But I'm just flagging it as historically EditorResourcePreview has been the source of many deadlocks due to it acquiring a lock on the renderer to make previews, at the same time where we're trying to import assets also using the renderer.

[akien-mga]

Likewise, this was also the issue with ProgressDialog and the reason for cmdline_export_mode to bypass both. See #85222 for details. This was a major pain during the 4.2 beta cycle, so we should tread with caution here.

CC @clayjohn

[mihe]

Right, yeah, I should probably have justified that particular if (!singleton->cmdline_export_mode) removal. The reason for removing that one was because I couldn't see why EditorNode::_save_scene_with_preview would be called during import/export. In the projects I've tested it never seems to be.

I don't quite see the relevance of #85222 with regards to progress_dialog though. The mutually exclusive part of cmdline_export_mode and progress_dialog seems to have originated from a tentative fix of #24031, which I suppose may still be relevant. Again, I didn't see any issues on my end, but the reporter there mentions discrepancies between Windows and macOS, so I suppose that could be why.

Anyway, I can just dial back this PR to just be about setting cmdline_export_mode from Main::start(). Having some sort of command-line output for --import seems more important than fixing the issue of the editor freezing up (temporarily) when not using headless.

[akien-mga]

Well I guess we can give this PR a try and see if anything breaks, and if so we can go back to a more conservative approach.

[mihe]

Also note that if #24031 is indeed still a problem, then that means it could potentially have been reintroduced in the context of --import, which is obviously live in 4.2.2 right now.