Extract metadata from manifests

[muffinresearch]

Here’s the information that is needed at the moment:

• guid
• type: extension, dictionary, langpack, search tool…
• name
• applications
• version
• homepage (optional)
• summary (optional)
• no restart (optional)
• strict compatibility (optional)
• target applications (eg Firefox, Thunderbird…)
• is multi package

[muffinresearch]

This can be split out into more than one issue if it makes sense.