The "Installation" dialog allows backing up the current installation by
creating a zip file from the .rockbox folder since quite a while. However, this
has the drawback that you need to update your build to create a backup, but
creating a backup might be desireable in other cases as well (before updating
themes, or just for backup reasons). Since the functionality is somewhat hidden
it's also not obvious to users such a functionality exists (most users are
likely to use the "Quick Start" instead).
Implement backup functionality as dedicated dialog placed on the Uninstall tab.
Rename the Uninstall tab to accommodate this.
Change-Id: I1d2c6c8f646672d1b66bb442408fbfc2eeec700d
