Fix/Changes: Combobox callback fixes/changes

[NickCloudAT]

With this commit comboboxes won't throw an error for a "nil" callback anymore. For some reason the game can not keep track of callbacks correctly anymore.

Also added a "oldValue" to the onSelect/onChange function of the combobox to ensure the previous set value is still sent.

I would happily have some people test this to see if I broke something or not and give their 2 cent on this.

Some things I'm not yet sure and maybe somebody has a better idea:

• Should the "oldValue" be nil if "self.selected" is not present or should we set it to the new value?
• I put "oldValue" as the last argument as to not break compatibility with perhaps already existing comboboxes.

Related issue: #1068

[ZenBre4ker]

Didnt look at your changes but I addressed similar stuff in #984, I believe. Gonna check that tomorrow.

[saibotk]

I have investigated this on my own and found the root cause for the issue.

Will open a PR to fix this without extra clean up functions, but why exactly are you adding the oldValue here? Do we need this?

[saibotk]

@ZenBre4ker actually you made the same mistake:
https://github.com/TTT-2/TTT2/pull/984/files#diff-a910efa517f7a2fbab8e7128c8091e74f3e0af9cc28e617e969c31612c317c60R104

Will write up the explanation in my PR

[saibotk]

Closed in favor of #1075

Thanks for investigating and writing this up!