`build`: Allow quickly changing targets.

[PerilousBooklet]

It would be very handy to have a way to quickly select a target for the current project, for example via a dedicated command that opens a commandview which lists all current targets and allows to select and run any one of them.
As suggested on Discord by Adam, the commands should be:

1. select
2. select + run
3. select + build

[adamharrison]

Should be fixed with d587194.