Embedded `Morphs` in `Text` should behave correctly with and without `Document`

[linusha]

Right now, we force Documents on Text with embedded Morphs. It would be nice to have them work without Documents as well.

This spans multiple different problem spaces:

1. [ ] It should be possible to add lines either via drag-and-drop in interactive mode and programmatically by including them in textAndAttributes. The second scenario should work with document and without.
2. [ ] Regardless of whether a Text has a document or not, its submorph-array should always contain its embedded morphs. They should also be correctly referenced in the embeddedMorphMap of the text.
3. [ ] When a morph A is embedded into a text, calling A.owner should return the text. This should work with or without document.
4. [ ] When a Morph gets removed from a text (either via drag-and-drop in interactive mode) or because the textAndAttributes of a Text were changed, the submorphs array of the text, its embeddedMorphMap and the owner of the previously embedded morph should be in a consistent state.
5. [ ] Regardless of whether a text is backed by a document or not, embedded morphs should update their positions correctly.

[linusha]

This is related to #763.