Add functionality for docks locking

Summary:
This patch provides mechanism for docks locking. This can be useful for some users and allows us to lock dock state during call Hide/Restore Docks action. Such locked docks don't change it's visible state during call. This provides behavior similar to QtCreator - we can lock for example left dock (with Project tool view) and quickly hide all other docks when it needed (after hiding unlocked docks the editor will be focused).

The patch allows user to control which docks should be locked by adding Lock the Panel from Hiding item to dock panel context menu. Lock settings are saved in global configuration file.

Test Plan: Tested with master branch.

Reviewers: apol, KDevelop

Reviewed By: apol, KDevelop

Subscribers: apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2958