Fix case where BaseTrackCache wasn't receiving dirty notifications if a
track begins life dirty.

Fixes Bug #1392917.
Only preserve legacy key text as user data if it is non-empty.

This was setting most tracks to dirty unnecessarily.
Move new end-of-track warning slider from the debug section up next to the other existing slider. Added a tooltip, changed the label as discussed in #352
Update translation template. Found 1826 source text(s) (14 new and 1812 already existing).