Make `codegenLanguage` be enum, not a checkBox in `GenerateTestsDialogWindow`

[EgorkaKulikov]

Description

Some time ago it was possible for user to select codegen language on UI, so the related setting was presented as a checkbox.
However, now language is set only in UtBot settings and just read from current state.

Variable codegenLanguage is still used in GenerateTestsDialogWindow (e.g. to customize parametrization enabled), but it is not used as a checkbox, as no UI control is shown to user. We may make this variable to be a kind of enum and make the code clearer.