Added various calls for model fields (getting & setting font, setting description)

[Aquafina-water-bottle]

This PR picks up the work from #362 and #363, and adds the following calls:

• modelFieldFonts
• modelFieldSetDescription
• modelFieldSetFontSize
• modelFieldSetFont

Additionally, some helper methods were added to prevent repeated code. So far, only the model field related calls have been changed to use the helper methods.

TODO:

• [x] Add tests
• [x] Verify tests
• [x] Test what happens if modelFieldSetDescription is used on older versions of Anki
• [x] Remove commented code
• [x] Add documentation to README
• [x] Final formatting check

Note:

• Currently, modelFieldFonts returns a json of the following format:

  [ { "fieldName1": { "font": "fontName", "size": fontSize } }, ... ]

  However, this deviates from the related modelFieldDescriptions call, that simply returns a list of descriptions. I was wondering if this is still fine?

[FooSoft]

@Aquafina-water-bottle I think that's perfect fine, and is probably in fact a better way of doing things.

[Aquafina-water-bottle]

Some additional notes:

• I changed the format of modelFieldFonts to return a dictionary format so fields can be accessed easier i.e.:

  {
    "field1": {
        "font": "Arial",
        "size": 20,
    },
    "field2": {
        "font": "Arial",
        "size": 16,
    },
  }

• In modelFieldSetDescription, it doesn't set the description if the key doesn't exist. This appears to be safe, and is properly tested in the tests. Additionally, when I made a new profile in Anki 2.1.49 and 2.1.54, the call still works as expected and properly sets the description.

[Aquafina-water-bottle]

This should be ready for review now! The tests that fail seem to be failing at random, so I believe it should be safe to merge if everything looks good.