WinRT: no read-only paths in QStandardpaths::writableLocation

As FontsLocation, HomeLocation and RuntimeLocation are read-
only on WinRT WritableLocation should return empty strings
in these cases. In addition all the other options were
added to the switch statement in writableLocation.

Task-number: QTBUG-38581
Change-Id: Iab994556844e713c6fa02028a0ec824ecb5ee82b
Reviewed-by: Andrew Knight <andrew.knight@digia.com>
Delete qml_plugin_import.cpp file only during distclean

For static builds of Qt Quick apps, qmake generates a qml_plugin_import.cpp
file. Just like the Makefiles, it should be removed only for distclean,
not in the clean step. This is what we do for non-qml plugins, too.

Change-Id: I5a3f2e7d27c3ffd5161162a8a03e4dd9c9245af5
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>
Fix QMutex documentation saying some function are static while they are not

qdoc only see a fake QMutex class (the same as the one built in bootstrap)
But that fake QMutex had static member while the normal QMutex class
has non static member.

QMutexLocker::mutex is also a const function in the real QMutexLocker

Task-number: QTBUG-38522
Change-Id: I220434ffc6a9e990029f770e2536ecb55b4e2182
Reviewed-by: Martin Smith <martin.smith@digia.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Add a comment stating QMutex::isRecursive should be made const in Qt6

Change-Id: I452b0764790112c59af77bc8d95f403ff37cbc4a
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>