[placeDisplay] Fix dnd regression

DND from the places section broke with 1c4c3afb when St.Label was
replaced with St.Button.
To fix, replace St.Button with St.Clickable and use a fake_release
when starting a drag.

https://bugzilla.gnome.org/show_bug.cgi?id=609218

data/theme/gnome-shell.css

@@ -516,10 +516,18 @@ StTooltip {
 
 /* Places */
 
-.places-actions {
+.places-item-box {
     spacing: 4px;
 }
 
+.places-item {
+    padding: 2px;
+}
+
+.places-actions {
+    spacing: 2px;
+}
+
 #placesDevices {
     padding-top: 4px;
 }