Annotations window

For PR https://github.com/MarkUsProject/Markus/issues/753

Taller textarea in new annotations window -- and also the ability to insert 2 spaces by pressing `TAB`.

Indentations are shown in the annotation hover boxes/the summary pane (apparently a CSS issue, since the browser automatically handles whitespace).

Code in annotations is properly displayed after editing.

Also moved the "Standby - Highlighting..." text into the locale files.