PixelPaint: Add menu items for Select All and Clear Selection

We also need to update the image editor when clearing selection
otherwise the last state of the selection will be displayed until an
update happens.

Userland/Applications/PixelPaint/Selection.cpp

@@ -56,4 +56,10 @@ void Selection::draw_marching_ants(Gfx::Painter& painter, Gfx::IntRect const& re
         draw_pixel(rect.left(), y);
 }
 
+void Selection::clear()
+{
+    m_rect = {};
+    m_editor.update();
+}
+
 }