New features & charater map improvements

• Fix wide char width handling and simplify in general via CSS vars.
• Use ES6 String.repeat to simplify char size measurements.
• Force height of all lines/chars to match to avoid glyphs drawing lines too
  high or low and making rendering overall inconsistent.
• Fix mouse move reporting (regression in hterm-1.65).
• Clarify modifiers in keyboard bindings.
• Invert touchscreen scrolling to match OS direction.
• Document hterm JS language (browser/runtime) requirements.
• Drop support for GR character maps (which have never actually worked).
• Add tests for hterm.VT.CharacterMap code.
• Clean up the hterm.VT.CharacterMap classes.
• Add reset & setOverrides helpers to hterm.VT.CharacterMap for customization.
• Add a new hterm.VT.CharacterMaps container class.
  API breakage warning: hterm.VT.CharacterMap.maps no longer exists. Any users
  of it will need to instantiate hterm.VT.CharacterMaps and use that API.
• Change mouse wheel scrolling when DECCKM is active to run only on the alt
  screen (and never the primary screen).
• Add support for custom notifications (iTerm2's OSC-9 and URxvt's
  OSC-777:notify module).
• Initial support for making virtual keyboards show up (for phones/tablets).
• Fix clicking of mailto: links.
• Fix ctrl+clicking in empty space (caused internal errors).
• Change Ctrl+V and Ctrl+Shift+V to invoke pasting directly instead of relying
  on the OS/browser to do so (makes macOS consistent).
• Include a terminal icon in all notifications.