#2983 Add toolbar button for "Always on top"

[Joshua Kinard]

Would it be possible to get a toolbar button + icon added for the "Always on top" item under the "View" menu? I use this feature a lot and it'd save a mouse-click if there was just a toolbar button for it. It could go right after the "Lock workspaces" icon.

I would suggest for an icon to use a thumbtack, but I went digging around on several sites for an appropriate Nuvola icon, but couldn't find one. The Nuvola author's original site in the help file is not valid anymore. Thanks!