Move common elements into /common/ and change imports

Elements work like they've always done, this just moves the commonly
used components away from the global namespace.

The qmldir file still does something, although at this point it is
unknown exactly how it's used and whether or not it could be changed to
simplify imports across the block.

src/main.cpp

@@ -425,7 +425,7 @@ int main( int argc, char* argv[] )
 
         QString path = QStandardPaths::locate(
             QStandardPaths::AppDataLocation,
-            QStringLiteral( "res/qml/mainwidget.qml" ) );
+            QStringLiteral( "res/qml/common/mainwidget.qml" ) );
 
         QQmlComponent component( &qmlEngine, QUrl::fromLocalFile( path ) );
         auto errors = component.errors();