Menu improvements

Editor CSS improvements

Some CSS improvements for the page editor, especially for smaller screens

Add default icon for editor buttons

Add a "plugin" icon to all plugin buttons

Configuration

First tests of making certain features configurable. For now, only the "dark mode" can be disabled in the options.

Image fix

corrected fill rule in SVG image

Editor improvements

Several improvements for the editor, including a resizable internal links browser, better dark mode and new icons