Dipole GUI interactivity and visual enhancements

[larsoner]

@wmvanvliet I played with the GUI a bit and have some tweaks to the UI:

1. Show GOF using twinx (could add a visibilite toggle if you find it distracting, but for me it's really useful to see)
2. Add y=0 line
3. Change color of dipole boxes to reflect the color of their lines
4. Add opacity sliders
5. Fix crufty print
6. Add time label attached to the vertical time line, remove it from the 3D plot
7. Colorbar optional in 3D plot, default off
8. Better brain opacity (old mode only set the brain-background opacity to 0.5, this now uses that plus an opacity of 0.3 on top of that for everything, so the blue dipole can actually be seen -- we might also / instead want the inner skull at some point)
9. Better UX -- show a splash screen, add a status bar for updates for slow operations, don't show window until geom is settled (no "flashing"), like mne coreg
10. Give more vertical space to the brain plot
11. Hovering dipole name bolds its trace
12. Tiny scraping fix for HiDPI

If you don't like some of these we can iterate more as needed!

main:

PR:

[payamsash]

Thank you @wmvanvliet and @larsoner for pushing this. If you agree after some improvements in this UI/UX, we could think about having consistent and similar interfaces for all mne GUIs (e.g. mne.viz.Brain, mne.gui.Coregistarion, and dipole)

[larsoner]

@payamsash yes that would be good! After this PR maybe you could bring over / generalize your improved "trace selector" widget to replace the dipole one (adding support for colors and hover that this PR adds)

[wmvanvliet]

Yes @payamsash, if you could take a look at the "Dipoles" box:

and make it as pretty as your "Traces" box in the source estimate plot, that would be nice!