PopupMenu: add horizontal sliders

Introduce Cairo-drawn sliders to be used in PopupMenus (for example for
volume). They are stylable to some extent (colors, border width, slider
height) and have the standard behaviour of a slider, except they are
completely modal (once you start dragging, all events are intercepted by
the slider, which thus is kept active and highlighted at all times).
They show numeric values between 0 and 1 (scaling must be performed outside)
and emit value-changed on button release, but no activate, keeping the
menu open.

https://bugzilla.gnome.org/show_bug.cgi?id=625029

data/theme/gnome-shell.css

@@ -148,6 +148,15 @@ StTooltip {
     height: 1em;
 }
 
+.popup-slider-menu-item {
+    height: 1em;
+    -slider-height: 0.3em;
+    -slider-background-color: #333333;
+    -slider-border-color: #5f5f5f;
+    -slider-border-width: 1px;
+    -slider-handle-radius: 0.5em;
+}
+
 /* Switches (to be used in menus) */
 .toggle-switch {
     width: 4.5em;