gui: store hidden status of buffers in layout (implements #152)

[maxcrees]

This allows buffers to be persistently hidden between sessions of
weechat if the layout is saved to the configuration.

[maxcrees]

As an additional note, this is especially useful in conjunction with a trigger like the following so one can keep buffers hidden indefinitely until they gain new activity:

unhide_buffer: line
               =? "${buffer.hidden} && ${displayed} && ${notify_level} >= 1"
               /1 "/buffer unhide ${buffer.number}"

[sim642]

Just to have this documented: PR #1412 and this PR would cause a conflict if both were merged. Both add a new field in the config file lines of layouts.

[maxcrees]

Maybe we can combine them into one PR such that active/zoom/hidden state is stored in one variable as a set of flags.

[sim642]

It's probably not worth the effort since in WeeChat's structs they're separate fields anyway, so correctly combining and splitting them is just extra effort. And also error-prone if one of those is changed in the future.

[maxcrees]

It would be more extensible if more things need to be stored in the layout configuration in the future though. I'm not sure what you mean by it being error-prone, though I have seen some people muck up enums on accident before...