Introduces left and right sidepanels to the text UI, including new source files, theme formats, and main window layout logic. Adds mouse support, activity-based coloring, and dynamic panel reservation. Updates build scripts and headers to support the new feature.
- completely removed the old textbuffer representation (
https://github.com/shabble/irssi-docs/wiki/Notes-256-Colour#textbuffer-encoding
)
- textbuffer-formats is an extra module, so if we unhook the signals it
should go back to the "old way" of storing pre-rendered tex
- design uses cache, original formats and list of arguments
