Draft: Unified Extensions Subsystem

[mjakeman]

Unify the local and web representations of Extensions

This allows us to use online data and fallback to local data when it is not available.

Practically speaking, it unblocks:

• Unified search across local/remote extensions (Search installed extensions #8)
• Accessing extension controls on the browse page (Feature Request: Show cog (settings) for installed apps in browse view #197)
• More information in the upgrade assistant, including installed status (Show in the upgrade assistant the percentage of compitable extensions that are enabled  #477)
• The UI redesign for 1.0 (UI Redesign (1.0) #67)

Currently, this removes the dropdown in the local extensions view as requested in the Circle review. However, I won't merge this until we make it instant otherwise it's a huge regression (and annoyance). We can do this by:

1. using local data
2. starting an async resolve
3. updating fields in-place with resolved data
4. caching this with a TTL of 1 hour

TODO for this PR:

• Implement functionality for browse page to open settings/etc
• Fix the offline details regression above^
• Find a way to still show error details without it being cumbersome
• Add Oscar to DOAP file 4900104
• Add Oscar to about dialog 38a5b57
• Remove commented out code
• Add license headers to various files 3cd970f
• Fix homepage link
• Fix merge conflicts
• Fix crash when opening non-EGO extensions (i.e. Fedora's Background Logo)
• Deduplicate code between Extensions Row and Details Page
• Add selection mode to bulk-uninstall extensions

What this looks like:

[mjakeman]

Rebased on master (with some breakages). Time to tackle that to do list... 🔨

[mjakeman]

Rebased again - now using property bindings for populating the details view.

It's still pretty broken, but we can bind the ExmUnifiedData directly to the Gtk widgets now. I'm thinking it'll be nice to have the ability to display local, fetch web in the background, then emit a property change notification once web returns and update the display. Will have to see what this looks like in practice.

Further steps could be finding a way to load the screenshot, icon, comments, etc via property binding and blueprint. It would be nice to e.g. declare an ExmImageResolver in the blueprint code and set the URL there and have everything magically work in the background.

[mjakeman]

And now supports remove/prefs on the details page:

Lots of duplicated code here, so will need some tidying up - but it works well!