Moved popup tab title handling from js to the python view.
Functionally, nothing changed.

Later on, we can use this to provide proper titles for the tabs.

Added translation for 'Tabblad' at the same time.
Added Afrikaans translation that came over with the translation update
Using first workspace item's name as the tab title.

So: per display group (=tab) you get the title of the first workspace item in
that group. As a fallback you get "tab 1", "tab 2".

The workspace item name is normally something like "Ground water wells" or
"Water level [mm]".
Updated changelog
Merge pull request #23 from lizardsystem/reinout-popup-tabs

Popup tabs with nice titles, hurray