Fix opening panel context menu when in panel config mode

Tobias Fellarequested to merge
work/tobias/panel-config-context into master

Without this patch, when right-clicking a panel while in panel config mode, the context menu would open, immediately close again, and exit the config mode.

This effectively reverts a workaround for an old bug in Qt. With the workaround removed, I still can't reproduce the described Qt bug. Beyond that, I can't judge whether the workaround is still relevant.

Relevant:

Before:

After:

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