QSettings: Prevent assert when passing empty keys.

[ChangeLog][Important behavior changes][QSettings]
QSettings::value() now returns an invalid QVariant
when passing an empty key. The code path ran into
an assert, which was only noticeable in debug
builds.

Task-number: QTBUG-41812
Change-Id: I5cc32be3aa267a132e9d6639ecd6cb0bbafc15b0
Reviewed-by: Stéphane Fabry, Cutesoft <stephane.fabry@gmail.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Export QFSFileEngine symbols

Although it's private API the symbols are used e.g. in the
Qt Installer Framework.

Change-Id: I557d3b86dbf87cb1b712bae09c3e8fecf6f15e67
Reviewed-by: hjk <hjk121@nokiamail.com>
XCB: Always set X window attributes in QXcbWindow::setWindowFlags().

QDockWidget and QToolBar set the Qt::BypassWindowManagerHint window flag
when unplugging and clear it in the endDrag() methods. This does not have
any effect since the attribute is not taken into account in
QXcbWindow::setWindowFlags(). Change the method to always set the attributes,
which should also make it possible to set/clear Qt::WindowTransparentForInput.

Task-number: QTBUG-41189
Task-number: QTBUG-38964
Change-Id: Id9eddc642489d18f44c7597f8fc1a1df71971306
Reviewed-by: Andy Shaw <andy.shaw@digia.com>
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>
Fix undefined behavior in QLoggingRegistry::defaultCategoryFilter()

Report by asan:

  READ of size 2 at 0x00000041dd40 thread T0
    #0 0x2af097b84da6 in QLoggingRegistry::defaultCategoryFilter(QLoggingCategory*) (lib/libQt5Core.so.5+0x566da6)
    #1 0x2af097b8387b in QLoggingRegistry::registerCategory(QLoggingCategory*, QtMsgType) (lib/libQt5Core.so.5+0x56587b)
    #2 0x4067f7 in tst_QLogging::QLoggingCategory_categoryName() tests/auto/corelib/io/qloggingcategory/tst_qloggingcategory.cpp:238
    <snip>

  0x00000041dd41 is located 0 bytes to the right of global variable '*.LC115' defined in 'tests/auto/corelib/io/qloggingcategory/tst_qloggingcategory.cpp' (0x41dd40) of size 1
    '*.LC115' is ascii string ''

At face value, memcmp("", "qt", 2) should not return 0, but since
the code invokes undefined behavior, the compiler can do whatever
it wants, including returning 0 here, further proving the fact
that there are *no* benign cases of undefined behavior.

Change-Id: I0c38622c47d1dcea450ea549370be1673b47b18d
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Move Qt Core examples under a common subdirectory

Qt Core examples were scattered into several subdirectories under
qtbase/examples. This caused an issue with the example manifest
file generated by QDoc; it expects to find all examples under a
common directory in order to produde correct paths to the example
.pro files. Qt Creator will not find the examples without a valid
manifest file.

This change moves the examples and edits the documentation files
accordingly.

Task-number: QTBUG-41963
Change-Id: I51d86782e0ba21c5c9bae5f15401ec774abe5cf8
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>
Android: Improve cache logic in findClass()

This change adds guards to ensure that we only do a class
look-up once when calling QJNIEnvironmentPrivate::findClass().
IF someone calls findClass() with an environment that does not contain
a "valid" class loader, we should fallback to loadClass(),
but only once.

Change-Id: If5fc82956db889f3269bb33c98a16c49cae55def
Reviewed-by: Yoann Lopes <yoann.lopes@theqtcompany.com>