Replace object visual by enum

[cx384]

This replaces the visual variable of the ObjectProperties class by an enum.

• It gets rid of some string comparison and can even use a switch statement, which is good, I guess, but I mainly want this for Only send changed object properties #15671.
• It still gets serialized as a string.
• A wanted to use an enum class, but replaced it with an enum, because it would need a lot of explicit int casts in order to use our EnumString. Also, I did not put it in a namespace instead of the OBJECTVISUAL_ prefix, since this way it is more similar to the existing code, e.g. the HUD enums. Tell me in case you prefer something else.

To do

Ready for Review.

How to test

👁️ 👁️

[cx384]

Thanks for the review, it should be fixed now.

[cx384]

Rebased

[nerzhul]

unfortunately another rebase is required. I'm totally in favor of such quality change.