A foundation for text editing and selection in Masonry

[DJMcNab]

This brings in a lot of the old work from Druid and Masonry (prior to linebender/masonry#56) on text, as well as some types from Glazier.

Needed work:

• Text display using abstract types
• Text selection with mouse
• Text input with keyboard
• IME integration (of the kind winit understands)

Follow up work:

• Keyboard control of the selection area (hard-coded hotkeys)
• Proper placement of cursor
• Input methods
• Proper hotkey handling
• Copy and/or paste

[waywardmonkeys]

Just a general comment that some things are prefixing with set_ and others with set_text_ and sometimes set_color is used in here and in other places set_text_color and it might be nice to make this more consistent. (Probably not immediately before RustNL...)

[DJMcNab]

Yeah, all of this API design could do with a tiny bit of cleanup; there's quite a lot of redundancy in these methods

[xStrom]

Approving in the interest of v0.1 for RustNL, with the understanding that the code introduced here is buggy and is missing functionality that will be addressed in future PRs.