IDE-235 Language change confirmation

[hannesweilharter]

In the language settings the user now has to confirm the selected language, and thus it's not possible anymore to accidentially change the app to a language the user doesn't understand.

• new confirm button in the language settings ListPreference
• new class "LanguageSettingListPreference" - derived from ListPreference class

https://catrobat.atlassian.net/browse/IDE-235

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