Display cells from hidden instruments in the timeline, using cyan shading, and maybe allow toggle hiding directly from timeline

I think we should keep the coloring scheme of the timeline simple, as it is currently, so all these things lasconic mentions of hidden staffs, measures, instruments, should all be colored in the cyan hue that is also used for notes outside the viewable range. If these are all covered by Measure::visible() then that makes the implementation simple.

One thing I should mention: the fact that there are so many ways that measures can be hidden, then I think that the thick cyan line separating the visible from non-visible now seems to me to be unnecessary, and might even be too distracting if the visible-nonvisible boundary forms odd shapes. So I would also suggest that we just get rid of that thick line entirely, so it isn't too busy on the eyes:

Interestingly, if right click a staff and set "Hide when empty" to always, then it seems that the timeline already does what is being asked in this feature request: it shades it in cyan.

I should also note that the timeline displays on a staff-per-row basis, not on an part-per-row basis. So regarding my feature to add a little eye icon for hiding, if it were to do the same as Edit->instruments, it should hide the entire part, not just that specific staff. Anyway, don't focus on the eye right now...focus on the shading first. I think there is more work to be done for multi-staff instruments anyway with the timeline (for instance by having them user a same rectangle in the first column).

The grey-green-cyan or whatever color it should be called has one nice thing going for it, and that is that it doesn't stand out too much. Sibelius uses orange for this area...and we found it stood out too much. Blue is specifically not allowed for these out-of-view cells because blue has a specific meaning of selection in musescore. Grey won't work because grey is currently used to indicate presence of elements (and using a lighter grey for these out-of-view cells wouldn't work because a filled out-of-view cell would look too much like an unfilled in-view cell).

If there is another color scheme that works with musescore let us know.

I'm still a little hesitant to have blue be used for visible area because blue should only mean selected.

There may be another way or shade. I also think that regardless of whatever hue or whether it colors inside or outside of viewable area, that the intensity should be very faint, since it is not important info, and so it doesn't distract. Again, I don't think the thick border between viewable and non viewable should be used since it is distracting and in fact Joshuas latest code no longer has that.

Maybe we should put the color in preferences text file (no need for gui, just the text tile), so people can try out different color schemes without recompiling.

yes of course I have used the Navigator a lot. But the navigator isn't used for selecting, while the timeline is used for selection, and so that is why I'm saying to leave blue for selections only. Keep in mind there are two ways currently for selection...range selections and simply elements selected. Range selected measures have a thick blue border and are shaded lightly blue, while element selected measures have a thin black border and are shaded lightly blue.

Again with the viewable border, I think you are missing an important point I've mentioned a couple times that the viewable border is potentially a very messy odd shape, not a clean rectangle.

Regarding inside and outside, it's the case of 6 eggs vs half a dozen. Just need to figure the way the doesn't conflict...the double blue is a bit conflicting but isn't too bad.