Progress on #22898: Support for voltas in GP5 files

This patch adds support for voltas in GP5 files (see issue #22898). Probably that bug should not be marked as fixed at this time as support is still needed for previous guitar pro versions, which work a little differently.

I've updated this pull request to 1) change the volta Guitar Pro test to remove additional staff types to bring that in line with PR #843 which has already been merged and 2) to use the 'auto' keyword (line 290 of importgtp.cpp).