PixelPaint: Draw the current editor selection as marching ants

This patch moves the marching ants painting code to Selection and
unifies the timer mechanism so that all marching ants are synchronized
which looks neat. :^)

Userland/Applications/PixelPaint/ImageEditor.cpp

@@ -20,6 +20,7 @@ namespace PixelPaint {
 
 ImageEditor::ImageEditor()
     : m_undo_stack(make<GUI::UndoStack>())
+    , m_selection(*this)
 {
     set_focus_policy(GUI::FocusPolicy::StrongFocus);
 }