The build system now supports platform-specific modifications to the
manifest for each build type. The need to customize the `id` on Firefox
motivated this change.
To support this, a new directory was made in each build type directory
for manifest changes. The images currently in this directory were moved
into an `images` subdirectory.
This new `manifest` directory can include each manifest file currently
in `app/manifest`. The `_base.json` file is assumed to exist, but the
platform manifest modifications are optional.