Ensure custom_slide_count always uses same total

[rschroll]

With a custom ending slide, the presenter and overview could give different numbers for the total slides. This fixes that by figuring the total number of slides directly in custom_slide_count.

[davvil]

That was actually intentional. As you are seeing all the slides at one time, I though having the global total was (slightly) better. I was doubting between both possibilities without a clear favorite, though, so I could be easily convinced otherwise.

[rschroll]

A few (weak) arguments in favor of the change:

• The user is not expecting anything about the page count to change by changing the view.
• The user may fear that the custom end page has been lost by switching to the overview.
• In the overview mode, the user already can see where the selection is relative to the total number of slides, so duplicating that info isn't as useful. What's harder to see at a glance is where they are relative to the marked end, so adding that info as text is more useful.

Of course, the current behavior isn't that bad, so I won't be upset if you don't make the change.

[davvil]

Ok, you convinced me ;-)