Use natural size for pathbar button

This fixes that the pathbar may disapper with long folder names.

This is an improvement of the nautilus commit 79157f5617886401379a724f8c526c430c4876a6, intended to entirely fix:
https://bugzilla.gnome.org/show_bug.cgi?id=313854
Merge branch 'pathbar_trunkate' of github.com:daschuer/nemo