Changes

Doesn't use a Java 1.8 Method to sort KeyframeList's contents

Revert "Doesn't use a Java 1.8 Method to sort KeyframeList's contents"

This reverts commit 7c342d8995fdcb103a9eb01e57efad15af66b427.

GuiElement interface's mouseClick method returns a boolean which is handled by ComposedElement to prevent mouse clicks on multiple elements at once
ComposedElement's entries are sorted so that GuiDropdown are always drawn on top
Modified thrown UnsupportedOperationExceptions in CustomImageObject to not be thrown if asset is null