gui: allow any Microscope Canvas to display FPS

It can be useful also for the SPARC
still hacky, but now at least, it's removed when the debug mode is stopped
gui: clean up StreamPanel

remove non needed subclasses

Seems like test cases pass
gui: some small fixes for the test cases

update to new code
at least now 2 test cases pass
model: don't fail on __del__ if the init failed
pvcam: whitespace cleanup
gui tests: update class name
gui: fix failure on starting when no backend
gui model: fix parsing wl polynomial in spectrum images
gui: ensure updates to the GUI are done in main thread

callbacks functions must have call_after
gui: fix the order of the stream entries in the stream bar
gui: fix order of images in SECOM

we reversed the convention that the SEM image should be last (was first), need
to update the code in SecomCanvas
gui: fix display of "visible" toggle button in acquisition window

actually, if the things are done in the right order, the stream panel takes care of everything
gui slider: fix bandwidth slider setting values outside of the range

float imprecision could lead to the values going slightly outside of the range