Detach button is not shown after a floating dock is docked

[bjbrandt]

Description

When auto-hide is disabled, the detach button in the dock's title bar is not restored correctly after detaching and re-docking the dock.

When the auto-hide feature is enabled, the detach button behaves as expected.

Steps to reproduce (Demo Application)

1. Disable auto-hide (e.g. by removing line 770 in demo/MainWindow.cpp).
2. Start the demo application and verify that the e.g. "Calendar 0" dock contains a detach button.
3. Detach the dock so that it becomes floating. The detach button disappears, which is the expected behavior.
4. Re-dock "Calendar 0".

Expected behavior

The detach button is restored when the dock is re-docked.

Actual behavior

The detach button is not restored after re-docking the dock.

[bjbrandt]

Note: The bug was maybe introduced by commit c1df05b which removed line 836 from src/DockContainerWidget.cpp. Re-adding this line seems to help