Supporting desktop themes via the content interface

[sergiusens]

Proposal: templates

In an effort to have a clearer way to express common patterns and reduce repetition, we’re proposing the introduction of the templates property. This is a root property as well as a per-app property (root templates apply to apps that do not declare a template). In a general sense, templates manipulate the YAML within the snapcraft.yaml, but only in a way that is expressible as a plain snapcraft.yaml. For example, they can add new parts, add a minimum set of plugs to apps, etc. Say a gtk3 templ…