feat: New setting: Rolling edit mode affects syllable conjoining

[lanye74]

In the timeline view, the rolling edit mode causes two adjacent words with no gap between them to act as if conjoined: pulling the end of one word forward in time will cause its sibling's start to move forward in time, such that there is never a gap between the words. When rolling edit mode is disabled, adjusting word boundaries solely affects that word.

However, when editing syllables, they always act as if rolling edit mode is enabled. This behavior can be manually overridden by pressing Alt to separate the syllable boundaries.

This PR introduces a toggle setting that allows syllables to also respect rolling edit mode, like words do. Namely: if the setting is enabled and rolling mode is disabled, syllables can be adjusted independently.

A note: This setting also respects alt-key inversion. While alt-key inversion is effectively useless in this context—since the user should be toggling rolling edit mode instead—I chose to leave it in order to mimic existing behavior and reduce logic changes :-)

demo.mp4

see conversation in discord

[lanye74]

This PR is built off of #143, which is why there are duplicate commits here. After you merge #143, then I will rebase and mark as ready for review/merge. ^^