Fix Hide/Restore Docks

Summary:
The patch fixes Hide/Restore Docks functionality. The logic of current version seems to be broken:

1. Open some left dock element
2. Open some bottom dock element
3. Hide docks with shortcut
4. Restore docks with shortcut - we see both docks, as planned.
5. Close bottom dock element
6. Hide docks with shortcut
7. Restore docks with shortcut - we again see both docks, but should see only left dock.

The patched version restores only docks, which was opened before hide operation.

Test Plan: Tested with master branch.

Reviewers: KDevelop, brauch

Reviewed By: KDevelop, brauch

Subscribers: apol, brauch, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2915