files-view: merge action and view menus

Usability tests conducted by Gina Dobrescu have highlighted a number of
issues with the toolbar menus. Users can't switch between list and grid
mode with a single click, and they have struggled to find the switch
between list and grid mode.

Allan Day has come up with a design to address these problems. The view
and action menus have been combined into a single menu, and we have
added a new button to the toolbar which toggles the view mode between
list and grid mode.

https://bugzilla.gnome.org/show_bug.cgi?id=764632