Allow to search for translation keys and it's value regardless of language or namespace

Add an input field in the navbar which allows to search for translations.

[kersten]

I have added this feature, with one restriction: The query will be fired with the current selected locale. This is a what I expect if I have selected a language.

To implement the search without a locale the locale combobox has to be extended to allow no selection, then all available keys would be shown for the selected namespace and the query would search for all key regardless of the language.

[kersten]

Another enhancement should be to only search the currently selected namespace. This makes sense for really large spaces. If no namespace is selected every single would be queried.