Update VE core submodule to master (5fed7c4)

New changes:
a554ea6 Add button to debug bar to enable input debugging
8480f41 Add 'valid' event to MediaSizeWidget
a1c42b5 Update OOjs UI to v0.1.0-pre (531d497d3b)
3910521 Prevent JS error in getNativeRange()

Change-Id: I5f13b5d5d0a15579ed2027a09e3fc3933346b2f7