feat: More in-keeping DevTools style

[matatk]

• Style the DevTools panel in a way that matches the browsers' DevTools appearance more closely, and makes more use of the available screen space.
• Support light and dark DevTools themes, including theme changes on-the-fly in Firefox.
• Streamline the base gui.html a bit due to the simplified styles, and demarcate the DevTools and non-DevTools parts of the UI in this file completely, rather than having to remove elements in JS after load.
• Streamline the build script a bit in light of the above.
• Also rename the "tag" used to demarcate stuff to do with the sidebar in gui.html and options.html from "ui" to "sidebar" for clarity.
• Add a bit of info abut the mutation stats at the bottom of the DevTools panel.
• Re-order the buttons/warnings disclosure in the DevTools panel to be more consistent.
• Style the buttons/warnings disclosure to look more interactive.
• Fix some layout issues in Firefox (using feature detection—thanks StackOverflow—this could be used for other styling too).

Fixes #405