topbar: Rename Done button and other fixes

Rename Done button to Cancel, and remove emphasis. Add text-button style
class.

This is to follow the new design for Content Selection Pattern at
https://wiki.gnome.org/GnomeOS/Design/Whiteboards/SelectionPattern

https://bugzilla.gnome.org/show_bug.cgi?id=706818
topbar: Use text-button style class on New button

This style class should be used instead of setting a custom size.

https://bugzilla.gnome.org/show_bug.cgi?id=706818
selectionbar: Use a standard toolbar

Use a standard toolbar at the bottom instead of an overlaid toolbar.
Add a revealer to slide up and down the toolbar.

This is to follow the new design for Content Selection Pattern at
https://wiki.gnome.org/GnomeOS/Design/Whiteboards/SelectionPattern

https://bugzilla.gnome.org/show_bug.cgi?id=706818
selectionbar: Exit selection mode after an action

Close selection mode once an action is performed. Always show
selectionbar even if there are no selected items.

This is to follow the new design for Content Selection Pattern at
https://wiki.gnome.org/GnomeOS/Design/Whiteboards/SelectionPattern

https://bugzilla.gnome.org/show_bug.cgi?id=706818
selectionbar: Break loop if already insensitive

If there is a machine that cannot be deleted while checking for
sensitivity of the Delete button, set sensitivity to false and get out
of the loop immediately.

https://bugzilla.gnome.org/show_bug.cgi?id=706818
selectionbar: Remove usage of stock items

Remove usage of stock items which are now deprecated.

https://bugzilla.gnome.org/show_bug.cgi?id=706818
selectionbar: Use text label for Pause button

Pause is an action item and string labels are now prefered for these
items.

This is to follow the new design for Content Selection Pattern at
https://wiki.gnome.org/GnomeOS/Design/Whiteboards/SelectionPattern

https://bugzilla.gnome.org/show_bug.cgi?id=706818
selectionbar: Add button style classes

Add "image-button" and "text-button" style classes to the buttons to
fix their padding.

This is to follow the new design for Content Selection Pattern at
https://wiki.gnome.org/GnomeOS/Design/Whiteboards/SelectionPattern

https://bugzilla.gnome.org/show_bug.cgi?id=706818
selectionbar: Fix selection bug on favorites

Selecting a machine not in favorites, while there is a machine that is
in favorites and selected, adds all selected items to favorites. This
happens because the active property is changed during the check for
selection, emitting a "clicked" signal.

This patch will block the "clicked" handler for favorite button while
changing this property.

https://bugzilla.gnome.org/show_bug.cgi?id=707076