Introduces quickzoom, new hotkey to toggle formants

[cwnaber]

Additions aimed at optimizing workflow when segmenting. Changes include:

1.) A new hotkey 't' can toggle formants on/off. This hotkey just calls the same code as when a user clicks the Toggle Formants button. No new functions were added.

2.) A new function expand_btw_ax_uev places the axis markers (green and red bars) just before the first user event and just after the final user event, then expands to those axis markers. This is intended for workflows where the UEVs already exist and are generally in the right place, but the user still wants to do some small-scale UEV adjustments. The function is modeled after the existing "expand" function, expand_btw_ax_markers . A new associated hotkey 'q' (quickzoom?) is included.

3.) Revised help text to include two new hotkeys.

[sbakst]

Worked for me! I tested on data that had been forced-aligned.

[cwnaber]

I agree on all accounts. 1.) Help text has proper carriage return after 'q' now. 2.) When there's one user event, 'q' will still quickzoom to relatively broad range (400 ms) surrounding that user event. If there's a specific workflow we're expecting to use that for, we can adjust that 400 ms value to something else. Otherwise I'll just leave it there for now.

[cwnaber]

I'm still learning what buttons do -- didn't mean to close and reopen the pull request!

[cwnaber]

Per convo last week:

• The "alignment checking" code stays locally in the new expand_btw_ax_uev (quickzoom) function.
• A new comment describes how the alignment checking code could be modularized.

Given previous consensus, we should be ready to merge now.