PixelPaint: Add ImageEditor::update_tool_cursor() method

This just allows us to ask the image editor to update the tool's
current cursor. This is useful in cases where a tool may want to
change it's cursor for some operation, and wants to invalidate the
active cursor cached in the ImageEditor.

Userland/Applications/PixelPaint/ImageEditor.h

@@ -36,6 +36,7 @@ public:
 
     Tool* active_tool() { return m_active_tool; }
     void set_active_tool(Tool*);
+    void update_tool_cursor();
 
     void did_complete_action();
     bool undo();