PixelPaint: Fix crash when started with path

This change fixes the initial tool selection when pixelpaint is started
with a path. Previously an already existing editor was expected when
the default tool was initially propagated - which was not the case if
pixelpaint was launched to directly load an existing image.

Userland/Applications/PixelPaint/ToolboxWidget.h

@@ -30,6 +30,7 @@ public:
     }
 
     Tool* active_tool() const { return m_active_tool; }
+    void ensure_tool_selection();
 
 private:
     friend class ToolButton;