- Refactored code to remove repetition and magic numbers

This reverts commit 6d38d7eb33.

This reverts commit 809252736b.

This reverts commit a71bda9adb.

When the AvailableModulesList is destroyed, it will wait for the current
module scanning job (running on a background thread) to complete. This
job will try to take the AvailableModulesList::lock when it completes,
so the lock must outlive the background thread.

Previously, when saving a temporary project, two native dialogs would be
created. The first was very short-lived, as it was automatically
dismissed by the second dialog. On older macOS versions, this appeared
to work correctly (the first dialog was so short-lived that it did not
appear to the user). However, on macOS 12.0, the second dialog fails to
display altogether.

Displaying two file chooser dialogs feels like a programming error, so
the code has been reorganised a bit to avoid this case.

When the result of find_if() is only compared to the container's end(),
the operation can normally be expressed more concisely using any_of(),
all_of(), or none_of().