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Details:  That hasn't worked in the last few months, since we changed
the QtVersion class from being passed by value to being passed around by
reference.

replace ExtensionSystem::PluginManager::instance()->getObject<Core::ICore>() by Core::ICore::instance()

forward declaration is sufficient.

The previous interface wasn't sufficient to properly support the Apply
button, since it didn't separate applying the changes from the cleanup
phase.

Designer settings pages still need some fixing.

Fixes Apply button causing problems for certain plugins, for example the
QuickOpen.

RevBy:    dt

Task:     237665
Details:  We parse the qmake output anyway, so we can simply add the
version from there. Seems to work nicely. Idea by hjk.

Task:     -
RevBy:    -
AutoTest: -
Details:  Add the include path for qplatformdefs.h to the fot.

RevBy:    Found by me.
(cherry picked from commit 58827feb)

RevBy:    Found by me.