Previously, margins were clamped to 0..9999, but negative margins are
well supported by most renderers. In addition, previously lua
automations automations were able to produce these negative margin
values, and they would be saved correctly. However, re-opening the file
would clamp the values, and they could not be edited in the edit box.
This commit changes the clamping to be -9999..99999, and allows entering
(and editing) negative values in all relevant fields. In addition, this
makes the subtitle edit box margin fields take 5 characters instead of 4
to accommodate negative numbers up to 9999 (also the reason for raising
the upper bound).
boost::split_iterator type-erases the predicate, which makes it require
a virtual call per character (!) along with a heap allocation. As it
turns out we only ever need one predicate (comparing to a single
character), so replace it with a split_iterator that just does that.
