Fix error disconnecting text view's line length margin handler.

A leftover from migrating from get_data/set_data to attributes.
Set conf.line_break.max_lines to three.

The demerit function should make sure three lines are used instead of
two in what would fit on two lines only in extreme cases.
Include gtkspellcheck version in debug dialog.
Fix atomic file writing.

In some weird cases, the subtitle file and its backup, although in the
same directory, can be on different filesystems causing os.replace and
os.rename to fail, in which case we need to fall back to non-atomic
file writing and just use shutil.move.

Adapted from Florian Léger's fork.
Emit less changed signals from the view.

It seems there's something slower in GTK+3 regarding updates. To fix the
worst case, emit one changed signal at the most when changing selection
in the view. This affects at least replace-all with lots of search
results and applying many text corrections.
Slightly speed up the float cell renderer.
Slightly speed up tree_path_to_row.
Speed up action updates by precomputing selected rows.
Use GLib.markup_escape_text after all, just in case.

It's not much slower.
RELEASE 0.21.1
Merge ../gaupol into gst

Conflicts:
	TODO