Error when using a group with the perspective extension
When trying to use the perspective extension with a group of paths, it will fail with:
```
Traceback (most recent call last):
File "perspective.py", line 138, in <module>
PathPerspective().run()
File "/inkscape-extensions/inkex/base.py", line 113, in run
self.save_raw(self.effect())
File "perspective.py", line 54, in effect
obj.path = obj.path.to_absolute()
File "/inkscape-extensions/inkex/elements.py", line 136, in __setattr__
super(BaseElement, self).__setattr__(name, value)
File "/inkscape-extensions/inkex/elements.py", line 290, in path
self.set_path(path)
File "/inkscape-extensions/inkex/elements.py", line 299, in set_path
"Path can not be set on this element: {} <- {}.".format(self.typename, path))
AttributeError: Path can not be set on this element: Group <- M -264.536 100.792 L -258.01 100.792 L -258.01 101.67 L -260.749 101.67 L -260.749 108.507 L -261.798 108.507 L -261.798 101.67 L -264.536 101.67 Z ...
<snip>
```
Example file: 
Tested with:
d3c9cd27 (Nov. 29, 2019)
In 0.92.4, groups can be used with the extension without a problem.
issue