Documentation toolview: separate find-text action from navigation actions

Summary:
Avoids a little the confusion about the Find button being for searching
in complete documentation.
Adding a more explicite menu text and tooltip should improve things
some more.

Reviewers: KDevelop, brauch, apol

Reviewed By: KDevelop, apol

Subscribers: kdevelop-devel

Differential Revision: https://phabricator.kde.org/D7166