A11y improvements, and keyboard/mouse tweaks

• Update to libdot 1.23. Build improvements, i18n helpers, and npm support.
• mouse: Support horizontal mouse scroll wheel events.
• keyboard: Add more keyboard binding callbacks.
• mouse: Disable default handling for mouse buttons 3 & 4.
• keyboard: Ignore Tab keys in onKeyPress_.
• etc: osc52.el: Fix terminal detection.
• build: mkdist: Delete dead code.
• a11y: Ensure the Page Up button isn't shown when the ScrollPort is focussed.
• build: Convert concat to a python script.
• a11y: Only announce a space character if it is the character actually printed to the screen.
• a11y: Mark offscreen rows that hold selection as hidden.
• a11y: Update the selection when accessibility is enabled even if it's collapsed.
• test: Remove invalid page-down-all-rows-visible test.
• a11y: Add announcements of the Space character.
• a11y: Handle announcements of selection changes.
• a11y: Simplify handling of duplicate live output.
• a11y: Change to have a role="log".
• a11y: Announce messages shown by showOverlay.
• a11y: Fix announcement of current scrolled percentage.
• a11y: Don't announce page up/down at the top/bottom of the screen.
• a11y: Fix an assertion failure in hterm.AccessibilityReader.
• test: Add simple test page.
• test: Add/update various test data files.
• a11y: Make it possible to enable/disable accessibility at runtime.
• a11y: Announce the current screen content on Page up/down.
• a11y: Add skip-links to allow screen readers to scroll the terminal.