#9181: fix crash trying to resume node editing

Clears temporal objects to avoid crash trying to resume node editing and
disconnects signals to avoid repetitive connects
Fix bugs in dxf symbology export
phonon needs backends
Dxf: another small improvement in symbology export
Dxf: store export map extent option
Merge pull request #1024 from ahuarte47/Issue_9181

Bug #9181: fix crash trying to resume node editing
Better conversion of QPainterPath to dxf
Improve dxf export for svg symbols
Remove legacy code
Remove drawEllipse from dxf paint engine (default implementation calls drawPolygon)
Export more simple markers to dxf
Fix circle position in dxf export
[TRANSUP] no by James

Signed-off-by: Werner Macho <werner.macho@gmail.com>
[Feature][Processing] Headless

To use QGIS Processing in python scripts out of QGIS.
The changes made concern the dependence to legendInterface. In cases
where the interface.iface is null (None) QGIS Processing uses the
QgsMapLayerRegistry instance.
Merge pull request #1031 from rldhont/headless_processing

Some changes to allow Processing to be run headless

It replaces legendInterface with QgsMapLayerRegistry, which is a better option
[processing] minor cosmetic change
[composer] Ensure that extent of new map items is centered on current centre of map