kill lineface_descr and move its fields into line_face

wattr{on,off} -> wattr_{on,off}

rename config_apply_color -> config_apply_style

add save_excursion and restore_excursion

allow changing the colors/attributes of the tabline

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