Fix user messages

I fixed the local storage service, user messages were jsut working fine.
I updated the UXD, they are now sticky to the right-bottom corner.
Adding overflow markers for preview frame scroller
New scroll overflow design (smaller teeths)
Fix scroller overflows apparitions
Fix background visual design
Update scroller overflows VD + fix zindex bug
Add canvas background switcher

A quick integration to make our minds on different backgrounds and ask
for feedback
Code cleanup + unsued images removal
Add 'visit gallery' action
Fix contextmenu right click (active only above drawing canvas)
Merge branch 'master' into gh-pages
Merge branch 'master' into gh-pages
Merge branch 'master' into gh-pages
Merge branch 'gh-pages' of https://github.com/juliandescottes/piskel into gh-pages