Add Flatpak base support (#64)

Adds the ability to define a Flatpak base to an app as part of a build.

Author: limdingwen

cookiecutter.json

@@ -12,6 +12,8 @@
     "flatpak_runtime": "org.freedesktop.Platform",
     "flatpak_runtime_version": "21.08",
     "flatpak_sdk": "org.freedesktop.Sdk",
+    "flatpak_base": "",
+    "flatpak_base_version": "",
     "finish_args": "",
     "modules_extra_content": "",
     "python_version": "3.X.0",

{{ cookiecutter.format }}/manifest.yml

@@ -2,6 +2,10 @@ app-id: {{ cookiecutter.bundle_identifier }}
 default-branch: {{ cookiecutter.version }}
 runtime: {{ cookiecutter.flatpak_runtime }}
 runtime-version: '{{ cookiecutter.flatpak_runtime_version }}'
+{%- if cookiecutter.flatpak_base or cookiecutter.flatpak_base_version %}
+base: {{ cookiecutter.flatpak_base }}
+base-version: '{{ cookiecutter.flatpak_base_version }}'
+{%- endif %}
 sdk: {{ cookiecutter.flatpak_sdk }}
 command: {{ cookiecutter.app_name }}
 build-options: