Bug #836299 - Revise Various Action Icons

Bug fix for https://bugzilla.mozilla.org/show_bug.cgi?id=836299.

Replaced the following icons with improved versions:

#	modified:   apps/email/style/images/icons/mark.png
#	modified:   apps/email/style/images/icons/move.png
#	modified:   shared/style/headers/images/icons/send.png
#	modified:   shared/style/headers/images/icons/send@2x.png