PixelPaint: Add Fit Image To View action

This is a feature I missed from Photoshop: it sets the scale and
position so that the image fits (it's longest dimension) into
the editor view. There's a 5% border left around the image to
provide context. This is just arbitrary seemed like the right
amount after some trial and error.

Userland/Applications/PixelPaint/ImageEditor.h

@@ -52,6 +52,7 @@ public:
 
     float scale() const { return m_scale; }
     void scale_centered_on_position(Gfx::IntPoint const&, float);
+    void fit_image_to_view();
     void reset_scale_and_position();
     void scale_by(float);