Proposal: Select installation target as the first thing

[ermo]

Motivation:

It immediately checks up front whether installation is even possible (both with the current requirements and in the future, where we will increasingly offer to partition and format the user's disk).

The target disk and disk layout is typically the biggest question mark for a user (including failures due to forgetting to format the ESP and XBOOTLDR partition), so why not tackle that first?

In comparison, the locale/timezone/keymap are typically straightforward customisation selections, which can follow once the more difficult/annoying stuff is out of the way.

Resolved by #45