Improving home screen customization

- Created a drawer with tabs for widgets, folders, shortcuts, wallpapers
(wallpapers are currently not implemented)
- Tapping outside the drawer dismisses it
- Moved the all apps icon to the upper right of the screen
- Adding a toast that says "No more space on screen" when dragging a widget to a full screen
- Fixed bug where you could stack two equal-sized widgets on top of each other on the homescreen (exposed by the new ability to drag widgets onto the home screen)

Change-Id: I03b65ce54a85d24328c94e0c06e249571de449ee
diff --git a/src/com/android/launcher2/CellLayout.java b/src/com/android/launcher2/CellLayout.java
index d69c56f..1d45565 100644
--- a/src/com/android/launcher2/CellLayout.java
+++ b/src/com/android/launcher2/CellLayout.java
@@ -1104,7 +1104,9 @@
 
             boolean found = false;
 
-            if (this.spanX >= spanX && this.spanY >= spanY) {
+            // return the span represented by the CellInfo only there is no view there
+            //   (this.cell == null) and there is enough space
+            if (this.cell == null && this.spanX >= spanX && this.spanY >= spanY) {
                 cellXY[0] = cellX;
                 cellXY[1] = cellY;
                 found = true;