[FEATURE] Migrate localization to a project settings window

[japtar10101]

Is your feature request related to a problem? Please describe.

Localization/Translation.

Describe the solution you'd like

It seems Unity is opening up the API to add windows under Project Settings and Preferences. In translation dictionaries and languages case, having a single Project Settings would be perfect, rather than awkwardly splitting it into ScriptableObjects.

Tasklist

• Look into which ScriptableObject should be moved into the Project Settings. Presumably, only the language list and the default translation dictionary needs to move into there.
• Update the TranslationString editor to allow not assigning the translation dictionary field, only if a default translation dictionary is already defined under the project settings.

Describe alternatives you've considered

The current methodology, which is to make everything ScriptableObject. The problem, however, is that the language list only needs to be defined once in a project.

Additional context

Carry-over from the old Bitbucket Mercurial project:
https://bitbucket.org/OmiyaGames/hg-template-unity-project/issues/64/migrate-localization-to-a-project-settings

Part of decoupling the Singleton scripts. See issue #19.

[japtar10101]

Fix #57 instead of this task.