Firefox fixes, openWindow security fixes, and few improvements

• Update to libdot 1.26. openWindow & new codec helpers.
• Optimize image display with Blobs & ArrayBuffers.
• Workaround touch focus bug on CrOS.
• Initialising iframe contents asynchronously on FF.
• Add support for new Clipboard API for copying text.
• Push selection logic down into copySelectionToClipboard.
• Fix paste event on Firefox.
• terminal: Stop passing down this.document_ when copying.
• Switch to array buffers for sending tty data.
• Provide readable pref strings.
• tests: Switch to a dark theme.
• concat: Stop listing test code in deps.
• Use new lib.f.openWindow helper.