Connect the Delete button
Save subtitles in their current order

Check and warn if numbers are not sequential.
Don't close subtitles before loading (as it clears the filename).
Set row count once -- this also preserves selection.
Fix deleting

This also fixes crash, when saving file after deleting one or more
items.
Add accelerators to buttons

Rename A, B, C, D into T1, T2, T3 and T4 respectively, to free up the
letters for accelerators.
Subtitles: add append flag to open()
Add Append menu item

And boot strap it.
Add accelerators to Close menu items
Boot strap Renumber button