A11y (screen reader) improvements & keyboard/mouse fixes

• Update to libdot 1.22. Unicode 11.0.0 updates and new helpers.
• osc52.vim: Split base64 output when using get_OSC52_DCS for screen.
• keyboard: Add media key handling only for Chrome OS systems.
• keyboard: Cleanly handle Firefox keyboard deviations.
• keyboard: Update Chromebook media bindings.
• build: Fix base64 usage on macOS.
• vt: Strip most control chars out for bracketed paste.
• keyboard: Improve addKeyDef documentation.
• mouse: Don't use user-configured word break matches when ctrl-clicking on urls.
• mouse: Double click should expand selection whether "copy-on-select" is on or off.
• a11y: Hide invisible elements from the screen reader.
• a11y: Add the aria-readonly attribute to the x-screen element.
• vt: Fix initial cursor display to match internal state.
• vtscope: Fix python-3 print func handling.
• prefs: Add Noto Sans Mono to the default font list.
• Clean up code a bit to pass linting.
• a11y: Add basic support for announcing command output to AT in hterm.
• a11y: Improve accessibility command output.
• tests: Add callbacks with image loading.
• terminal: Preserve selection when scrolling collapsed selections.
• Crush all images.
• tests: Fix floating point rounding error in Terminal.dimensions.
• vt: Unbreak legacy mouse wheel encoding.
• vt: Implement OSC 104 (color reset) support.
• vt: DECALN: Fix cursor position.
• prefs: Improve descriptions and translation tips.