Fix for LAME flags to take effect immediately

Because the service constructor was fed a string from the config object
for the LAME flags, the state of the string obviously didn't change when
the options got updated.

Changes this parameter to be a callable (at least for now).
Fixes for missing input/output files

Adds additional checks for when the output file a service is supposed to
generate is missing or an intermediate file isn't in-place as expected.
Fix to avoid reading sound tags within legacy tags

Adjusts the regular expression for legacy tags such that we cease
consumption as soon as we find a square bracket of either kind -- this
means that if we encounter a sound tag within a legacy tag, we stop
processing the text so that we do not accidentally read out a filename.
Fix to remove window help button on Windows

We do not offer this sort of per-widget help on any of our dialogs, so
we can safely turn this off.