latex: Added toggle function and use it in menu.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
latex: Added `weechat-latex-preview-region'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
latex: Auto mode and improved menu.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>
latex: Add `weechat-latex-preview-line'.

Signed-off-by: Rüdiger Sonderfeld <ruediger@c-plusplus.de>