Fix compilation with Qt4

[VictorLamoine]

Fixes #1369

This removes text eliding in dock widgets because the FindDirectChildrenOnly flag is only available starting from Qt 5

@theobslhc can you test compilation with Qt4?

[theobslhc]

@VictorLamoine thank you !
Compilation works fine with Qt4.
No problem at runtime with the configuration file linked to in PR #1168.

[rhaschke]

This solution essentially reverts #1168 for Qt4. To make the change less intrusive, I suggest to mask out only the content of onDockPanelChange but keep all other code as is. This will also maintain ABI and API compatibility.
Even better, you should mimic FindDirectChildrenOnly by checking child->parent == this ;-)

[theobslhc]

Thanks @rhaschke for the suggestion. Here's another PR : #1371