#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
[relations] No crash when clicking remove with no selection
Fix #9224
[relations] Fix autogenerated feature forms
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
Add the expression for the diagram attributes