scanview, xrf_plot, etc uniformity

[newville]

scanview, xrf_plot, GSEMapViewer, (and other "rich" GUIs meant to be launched from the larch command-line) should have a more uniform interaction:
a) not block (scanview currently creates an App, which blocks)
b) allow no argument version that simply raises GUI, from which data can be read.
c) fully use current-interpreter, so that namespace is shared.
d) offer raising of larch-gui.
e) mapviewer should be launchable from command-line

[newville]

partial success ...