Add setting to make fold state persistent

[snoopy]

@orbitalquark , what do you think about adding a setting to make the code fold state persistent across restarts?
I think this might be doable as a module but maybe it could just be part of core TA.

[orbitalquark]

I have thought about this, but file contents can change between Textadept sessions (e.g. git pull). That could mess things up, so I didn't pursue it further. What do you think?

[snoopy]

Good point. Maybe disregard any saved fold state if the line no longer has a fold header?

[orbitalquark]

I did run into another concern that files will need to be lexed when opened in order to detect fold points. This could slow down startup, but we can selectively lex if there are lines to fold. Perhaps that won't be so bad.

I've added this feature in ef93085.

[snoopy]

Nice, thanks.
What about adding a setting for this so users can decide if they want to take the performance hit?

[orbitalquark]

No, I don't think it's necessary right now. We can always revisit it if it truly becomes a problem.