Smyth's Grammar plugin

Works on #161 
Closes #115 

First implementation of a Smyth's Grammar plugin. It's not finished by any means, but this PR gets larger and larger - also contains some things that are probably unrelated to the SG plugin itself, therefore we merge early and revisit the task in other PRs.

Contains:
- Selection logic for SG categories
- Styling of texts in plugins (playing with font-sizes e.g., led to #184)
- Refactor to the `nestedMenuCollection` and `nestedMenu` directives (not 100% done, will do a separate PR for that)
- Final removal of `interjection` as part of speech for Greek and Latin
- Context menu for SG plugin and general fixes to these menus

What's still missing:
- Need to be able to save SG information
- Need to import data from the XML source of the Grammar by Perseus