QmlDesigner.DragTool: Remove import handling code

We have an import manager now.

Change-Id: If0123059a0fa1b51ef91c13bf24e3e6998195880
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.ItemNode: remove check for QtQuick imports

The Qt Quick Designer requires a Qt Quick import, so we
do not have to check for this case anymore.

Change-Id: Icbfa8fcf97eb43a503405e508e4afe1a16cc1542
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.ItemNode: Remove import handling

We have an import manager now and do not have to handle imports
in the code for item creation. This is just a possible source of bugs.

Change-Id: I5de40b874659f27f0f1a8d83dd78bcae2194aa32
Reviewed-by: Marco Bubke <marco.bubke@digia.com>
QmlDesigner.ItemLibrary: remove forceEntry

This feature allowed items to show up in the library even if the import
was missing. This feature was error prone and increased the complexity
of creating items a lot. Since we now have an import manager this is not
required anymore.

Change-Id: Ia127077be0b6c82a047cbb8f86ac3ab24e2d82ae
Reviewed-by: Marco Bubke <marco.bubke@digia.com>