Customisable lane width for ELNs

Part of #798 (closed)

Section definitions should also take a a_eln annotation. In the GUI this should be used to mark the width of a lane with SectionEditor in it. The width value should be interpreted as a "React css value", i.e. the number 700 as well as strings like 700px or 70% should work.

class MySection(MSection):
    m_def = Section(a_eln=dict(lane_width='70%'))