Quick access to tools volume

[tom-seddon]

Covering some aspects of #82.

Could the ROM trap Shift+B+<something else>+Break?

Could have it do something specific like *EXEC ::BEEBLINK:0.$.!BOOT and then make all the tools code use paths with volume names. Now the existing volume doesn't have to be unmounted.

[tom-seddon]

If detecting 2 keys proves a problem, pick another one from the key list here: https://beebwiki.mdfs.net/Break_key

Ideally it'd auto boot without SHIFT...