New TextWidget, throbber widget, documentation

This merge request rewrites TextWidget. Functionally, it's the same. The code is just slightly cleaner and easier to maintain, better-documented, and there have been some important fixes.

Letter wrapping (word breaking) has been added. If a word is too large to fit on a single line, it will be split into multiple words. Resolves #2 (closed).

Most of Ritchie.Graphics now has API documentation.

Themes can now specify the text color for <link> tags, and can specify whether links shall be underlined.

Throbbers have been added to the engine. It is up to the theme to actually render them. Toolbox Editor, and Socially Distant, render a throbber based on the famous system clock animation from the PlayStation 2.

Images inside rich TextWidgets now get assigned image descriptions. For the markup <img="/path/to/image">This is a picture of a cow</img>, the image will be assigned an image description "This is a picture of a cow." Markup in image descriptions is stripped.