app-well: Set menu width

Currently the app well menu has unrestricted with which can cause it to grow
with long window titles and can even go offscreen that way.

So set a max size, long titles will now be properly elipised.

https://bugzilla.gnome.org/show_bug.cgi?id=738054

data/theme/gnome-shell.css

@@ -232,6 +232,10 @@ StScrollBar StButton#vhandle:active {
     border-width: 0px;
 }
 
+.app-well-menu {
+    max-width: 400px;
+}
+
 /* The remaining popup-menu sizing is all done in ems, so that if you
  * override .popup-menu.font-size, everything else will scale with it.
  */