add pseudo-classes image sorting

Pseudo-classes priority: link > visited > hover > active

Before sort:

[
  dah-active.png
  dash.png
  dash-active.png
  dash-hover.png
  dash-link.png
  dash-visited.png
] 

After sort:
[
  dah-active.png
  dash-link.png
  dash-visited.png
  dash-hover.png
  dash-active.png
  dash.png
]