1.6.0: GTK+3 theme improvements

 - refactor linked buttons;
 - fix notebook tabs;
 - better theme performance with dark colorscheme;
 - wm borders fallback logic;
 - dynamic theme preview assets;
 - tests improvements.

fix(gtk3.20: button): replace .horizontal to :not(.vertical)
feat(gtk3.20: button): disabled button fg
fix(gtk3.20: entry): make entry the same height as button
refactor(gtk3.20: button): adjusting inset box-shadows
feat(gtk3.20): improve linked buttons, spinbuttons, stackswitchers and so on
feat(change_color; gtk3*; metacity): integrate SVG preview asserts by @nana-4
fix(gtk3.20: button: spinbutton): fix borders once again
feat(gtk3.20: global): choose wm title color based on cotrast
fix(gtk3.20: notebook): fix 'jumping' tabs on hover
fix(gtk3.20: menu): generate border and other colors based on menu_bg/fg
feat(gtk3.20: button): compute button outline radius based on common border radius and button outline offset
feat(maintenance_scripts: recent history): add option to hide non-functional commits