Refactor, mel.lib.ui.LeftRightDisplay: publish show()
Refactor, mel.lib.ui.LeftRightDisplay: publish show()

Support supplying custom content which is refreshed on an ad-hoc basis
by publishing the show() method on LeftRightDisplay. When the custom
content is changed, then show() should be called in order to trigger a
call to _get_image().

Test Plan:
$ ./meta/run_tests
.. Exercise functionality in later commit ..

aevri/mel,aevri/mel