use term default color by default for every line type

allow customizing left and right padding + use_default_colors

allow changing the background color of the body window

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!

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

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.

disentangle cmd_scroll_line_up/down and ncurses cmd_scroll_line_up and down are the fundamental functions called every time there's a need to scroll the window. cmd_scroll_up/down calls them in a loop, cmd_next_line and cmd_previous_line calls them too. And so on. These two functions, historically predates some of the abstractions of the `struct buffer', and handle ncurses directly using wscrl and print_line. However, since dispatch_stdio calls redraw_tab anyway, there's no need to update the window content here. (Well, wscrl + one print_vline *may* be a bit faster than the loop over print_vline in redraw_window, but we can optimise the latter later.) This also (should) dis-entangle the cmd_* functions and ncurses completely \o/

style

added olivetti-mode (the function)