Update sidebar.html button name

Updating sidebar.html to call out that contacts are in fact "Google contacts"
replace the clock icon by a proper contactlist icon
Replace old icons with the new ones
Merge pull request #402 from ametaireau/contactslist-icon

Replace the sub-panel icons with proper icons. r=Standard8
Merge pull request #401 from ppapadeas/patch-1

Update sidebar.html button name