This splits the platform creation into 3 parts:
 * base - create the initial directory based on the parent platform
 * prepare - run prepare commands and apply all changes
 * cleanup - apply cleanups and cleanup commands

This has cacheing advantages in that prepare_commands and cleanup changes
only cause the minimal amount of rebuilds.

Additionally, it ensures that the mtimes are zeroed out (from the
previous checkout) both when the prepare and cleanup commands are run.
This is actually important, since these often generate caches (for
example fontconfig ones) which rely on zeroed mtimes so they match
what will be deployed.

Closes: #277
Approved by: alexlarsson

Closes: #283
Approved by: alexlarsson

If we are spawning applications on the host using the Development service,
then we want those commands to exit when the flatpak-builder process
exits, as can happen from Ctrl^C or kill().

By using a static FlatpakHostCommandFlags we only ever check this a single
time and then each subsequent request will do the right thing.