v0.5.0: m Tag tabs with a colour from the tab menu

Added

• Tabs can be tagged with a colour from their right-click menu (Colour:
  red, yellow, green, cyan, blue, purple, or None), taken from the theme's
  palette. A tagged tab has a stripe along its edge in the sidebar or bar,
  and a ring around its number in the collapsed sidebar, which still shows
  when an alert fills the number.

Fixed

• Backspace works in the search bar (Ctrl+F); it did nothing before.