Allow scaling documentation view (BUG 285162)

Summary:
Add ZoomController utility

Prevent future StandardDocumentationView ABI breakage

Implement Ctrl+mouse_scroll zoom in StandardDocumentationView

Eliminate a quite restrictive hard minimum documentation font size limit

Add ZoomActions utility

Show zoom actions in StandardDocumentationView context menu

Add default context menu actions getter to StandardDocumentationView

Test Plan:

built, installed;
verified that Ctrl+mouse_scroll and context menu actions work;
verified that all the previously available context menu actions are preserved;
passed 3 related tests: test_cmakemanager, test_manpagemodel, test_qthelpplugin.

Reviewers: KDevelop, mwolff

Reviewed By: KDevelop, mwolff

Subscribers: aacid, kfunk, rjvbb, mwolff, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D3040