Do not cancel annotation mode after highlighting

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
3 jobs for 862-permit-continuous-annotation in 15 minutes and 59 seconds (queued for 116 minutes and 39 seconds)
latest
Status Job ID Name Coverage
  Build
passed #81979
flatpak

00:10:44

manual #81978
allowed to fail manual
gtk-doc
passed #81977
master

00:05:14