Styles: add drag-accept style to icon groups

data/Application.css

@@ -126,6 +126,22 @@ icongroup.running:active .icon-group-bin {
         transform 250ms cubic-bezier(0.4, 0, 0.2, 1);
 }
 
+icongroup:drop(active) .icon-group-bin {
+    background: alpha(@base_color, 0.5);
+    box-shadow:
+        inset 0 -1px 0 0 alpha(@highlight_color, 0.2),
+        inset 0 1px 0 0 alpha(@highlight_color, 0.3),
+        inset 1px 0 0 0 alpha(@highlight_color, 0.07),
+        inset -1px 0 0 0 alpha(@highlight_color, 0.07),
+        0 0 0 1px alpha(@borders, 0.3),
+        0 1px 1px alpha(@borders, 0.2),
+        0 1px 4px alpha(@borders, 0.4);
+    transform: scale(1.1);
+    transition:
+        box-shadow 300ms cubic-bezier(0.4, 0, 0.2, 1),
+        transform 300ms cubic-bezier(0.4, 3, 0.2, 1);
+}
+
 .reduce-transparency icongroup bin {
     background: @base_color;
 }

src/WorkspaceSystem/DynamicWorkspaceItem.vala

@@ -72,9 +72,11 @@ public class Dock.DynamicWorkspaceIcon : ContainerItem, WorkspaceItem {
 
     public void window_entered (Window window) {
         image.gicon = window.icon;
+        set_state_flags (DROP_ACTIVE, false);
     }
 
     public void window_left () {
         image.icon_name = "list-add-symbolic";
+        unset_state_flags (DROP_ACTIVE);
     }
 }

src/WorkspaceSystem/IconGroup.vala

@@ -58,9 +58,11 @@ public class Dock.WorkspaceIconGroup : BaseIconGroup, WorkspaceItem {
         }
 
         additional_icons.append (window.icon);
+        set_state_flags (DROP_ACTIVE, false);
     }
 
     public void window_left () {
         additional_icons.remove_all ();
+        unset_state_flags (DROP_ACTIVE);
     }
 }