gui: use pkg_resources for finding the resource file

avoid using our own trick implementation.
config: update delphi config file for precise excitation wavelengths

the original values were read out of a PNG graph. Now they come from the official values measured by Lumencor.

The main advantage is that it should allow the GUI to display exactly the same values as written in the specification.
gui: guess the emission or excitation wavelength when multiband

Using the other wavelength, it's possible to do an informed guess.
Useful if no dye was selected, then the colour of the emission and stream tint is more likely correct.
Merge branch 'master' of vpn.delmic.com:odemis