Add a style editor to NXTheme

There is currently no easy way to modify the style of an element in NXThemes apart from editing the XML theme setup file..

A style editor would for each element:

• show a preview of the rendered element (wysiwyg), starting with fragments, since cell, section, and page styles should probably be edited using a more straightforward type of UI (e.g. 'set background', 'set borders').
• allow to select an area of the rendered element's markup. This becomes a CSS selector (e.g. "ul.menulist li a")
• display a form containing the style properties for the selector
• allow to select preset properties (there is already support for property presets in NXThemes, but no UI to select them).
• save the changes
• update the preview.