[Azoth] ChatTabWebView::chatWindowSearchRequested() for #1745.
[Azoth] Handle chatWindowSearchRequested() for #1745.
[Azoth] Removed excessive include.
[Azoth] Added "Highlight occurences" action.
[Azoth] Added "Highlight the nickname" action.

Probably closes #1745, if that would be enough without supporting
highlighting multiple nicks.
[Azoth] Slight code style fix.