popupMenu: Hide separator label when not in use

The optional label support introduced in commit af063dc2f2ec broke
the centering of separators, as the label still adds additional
spacing even when empty.
Properly hide the label actor in that case to fix the alignment.

https://bugzilla.gnome.org/show_bug.cgi?id=730753
Support (fallback) app menu in SSD

We now allow "appmenu" in the button layout to make synchronizing it
with GTK+'s client-side decorations easier, but as some people tweak
their settings to get in-window app menus even when using the shell,
actually pop up the app menu when the button is activated.

https://bugzilla.gnome.org/show_bug.cgi?id=730752
theme: Bring fallback app-menu style closer to GTK+

The fallback app-menu in GTK+'s client side decorations obviously
uses the GTK+ theme rather than the shell one; update the style
of our own fallback app-menu to resemble that style.

https://bugzilla.gnome.org/show_bug.cgi?id=730752
windowMenu: Ping window when activating any menu action

We do this for actions in the regular app menu, it makes sense to
do the same for window menu and fallback app menu.
viewSelector: Don't re-navigate into the active page

Starting keynav into the active page is handled from a key-press
handler on the stage, however we should not "start" keynav when
we are already navigating elsewhere - the latter can happen when
keynav fails (for instance because the focus is trapped inside an
open app folder or at the end of the dash), and the event bubbles
up to the stage. So make sure to only handle the event to actually
start keynav, to not interfere with the normal navigation handling.

Thanks to Carlos Soriano <carlos.soriano89@gmail.com> for the
debugging footwork.

https://bugzilla.gnome.org/show_bug.cgi?id=726760
windowMenu: Only add workspace actions when enabled by session mode

When workspaces are disabled by the session mode, all workspace
related actions don't make sense. Worse, the "Move to Workspace ..."
actions allow sending windows into a nirvana with no means to
get them back.

https://bugzilla.gnome.org/show_bug.cgi?id=730653