Leave annotation button selected to keep annotating a document

Previously, if user wanted to highlight multiple text segments, she needed to press the highlight button before each highlighting.

This commit keeps the annotation mode enabled until user cancels it manually, allowing a better highlighting flow.

https://bugzilla.gnome.org/show_bug.cgi?id=789888

https://bugzilla.gnome.org/show_bug.cgi?id=790618

Closes #862