Always show two buttons in pathbar

The pathbar is pretty useless if there is only on button for the current directory.

This is a cherry-pick of 3cf2ca8ae26ca39534d26237dd25a756060617e0
from git@github.com:daschuer/nautilus.git
fixed scrolling with two buttons
pathbar: do not shink buttons below minimum
pathbar: display one button if two minumum buttons do not fit
Merge branch 'master' into pathbar_two_button

Conflicts:
	m4/gtk-doc.m4
	src/nemo-pathbar.c
Pathbar: make better use of Space in the two button shrinked state
Merge pull request #466 from daschuer/pathbar_two_button

Pathbar two button