It was possible that the update method was called recursively. This
could result in unpredicted behavior and in the worst case a crash.
This was exposed by adding tooltips to the window decoration (master).
There we have the following sequence:

1. pointer above an internal window (m_internalWindow is not null)
2. updateDecoration is called
3. triggers a hover enter on a decoration button
4. this creates a Tooltip
5. which means a new internal window is added
6. update is called again
7. m_internalWindow gets reset
8. back to previous update, now m_internalWindow is null although the if before evaluated to not null

-> crash

To prevent this a recursion blocker RAII class is introduced which
calls update again if the method got entered a second time.

Thus the update method can finish operation without the assumptions
being changed while executed. The changes which are required due to
the second call will be applied once we leave the method.

Targeting 5.12 as this is a general problem, although the bug is
only in master.

BUG: 389350
FIXED-IN: 5.12.1

No more crash when hovering close button on DebugConsole