MRG, MAINT: Unify 90% similar code

[larsoner]

plot_source_estimates and plot_vector_source_estimates share 90%+ of their code. This unifies the two approaches into a single _plot_stc function.

This will make things like #8064 much easier, as instead of making deep changes in two places, you can make them in one.

[agramfort]

@GuillaumeFavelier i let you merge if happy

[GuillaumeFavelier]

Thank you @larsoner !