rework color management: introduce the trailing props now, instead of prefix and line style, I'm adding the trailing style too. This changed the configuration syntax too.

improve colors when olivetti-mode is enabled

first scratch for colour support!

kill empty line

add enable-colors (variable) currently unused, will help later when adding colour support

work around some GCC warnings clang doesn't emit any. To be honest, some of them are actually right (like the missing `void'), others are impossible but still it's not a bad idea to initialize some variable. Future me maybe will be thankful.

detect tab switch and force redraw in case

bring back the ugly hack for the side window

improve variable section

sync

fix end-of-buffer glitch before, the cursor went one line after the end and glitch everything.

fix cmd_scroll_line_up buffer->current_line *IS NOT* the first line at the top of the window...

kill ui_notify and export {v,}message

move all cmd_* functions in their own file this detangles definitely the commands from the ui. Various variables and struct from ui.c needed to be exported, aside from that, no functional changes.

optimize redraw_window don't redraw the page if no scrolling happened. There's still need to ``force'' the redraw sometimes, like after a resize or after toggling olivetti-mode, so add a buffer->force_redraw flag for it.