Emit signals from icon border when no mark was clicked

Summary:

• currently it is not possible to get a signal from the icon border menu when no mark is clicked, which prevents the creation of custom context menus for no marks
• would probably break programs which set handled to true in any case without checking the mark type

Test Plan: - tested the change with KDevelop and observed no change

Reviewers: KTextEditor, dhaumann

Reviewed By: KTextEditor, dhaumann

Subscribers: dhaumann, kwrite-devel, Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D6541