tooltip: Add alignment options.

It's a list of preferred alignments because sometime the best one wont
fit in the screen.

lib/awful/tooltip.lua

@@ -124,6 +124,7 @@ local function apply_outside_mode(self)
     local _, position = a_placement.next_to(w, {
         geometry            = self._private.widget_geometry,
         preferred_positions = self.preferred_positions,
+        preferred_anchors   = self.preferred_alignments,
         honor_workarea      = true,
     })
 
@@ -326,6 +327,18 @@ function tooltip:set_preferred_positions(value)
     set_geometry(self)
 end
 
+
+function tooltip:get_preferred_alignments()
+    return self._private.preferred_alignments or
+        {"front", "back", "middle"}
+end
+
+function tooltip:set_preferred_alignments(value)
+    self._private.preferred_alignments = value
+
+    set_geometry(self)
+end
+
 --- Change displayed text.
 --
 -- @property text