New lines have to be committed before they can be selected.

Closes #1541.

wxRemoveFile notifies the user of errors via a dialog box, and errors
when cleaning the cache are normally just due to two copies of Aegisub
trying to clean it at the same time.

Truncate syllables which extend past the end of the line rather than normalizing all of the syllable lengths

Some of the debug iterator code seems to not like 'delete map[key]', so
change things a little to do things in a way that appears to be fine.

Actually use the limits set in the preferences dialog rather than always
limiting the MRU lists to 16.

Currently only the Find and Replace limits can be set to above 16 due to
how the open recent items commands are implemented.

Closes #1528.

Instead, save a single wav spanning the full range of all selected
lines.

Knocks 70 KB off the total size.

This is a potentially sensible operation (to convert from VFR to/from
the video's average FPS), so there's no reason to require that the input
and output timecodes be from different places.

Closes #1530.