Sort and group theme properties in docs, improve formatting for theme and enums

[YuriSizov]

This PR makes sure that theme properties in the documentation always follow the same order: they are grouped and ordered by their data type (color, icon, stylebox), and then sorted by name. This affects both the built-in help and the online docs (because order in the XMLs have also been updated).

Additionally, both theme properties and enums/constants have received an update to their look and their description is now on a new line. This is required because a lot of those entries have rather large descriptions which are hard to read otherwise.

Here's how the theme properties look now:

A good example of why it's needed for enums as well:

And a few comparisons:

• A short enum: Before | After
• A mixed bag: Before | After
• A very long but succinct list: Before | After 1, After 2

[dalexeev]

Maybe it would be better to make a table with columns Name, Value, Description (without headings)? Then long enum with short descriptions will not take up more space in height?

[YuriSizov]

That won't be very usable for long descriptions, namely because the width of the content is somewhat fixed. Plus, some enums are a mix of both cases, and in-betweens too.

PS. We are looking into improving the workflow for these help pages. Something that would help with them being stretched vertically (which is a problem as is).

[akien-mga]

Thanks!