initialize remote data in one go

Previously each extension was fetched separately from the API, this
fetches all installed ones in one go, speeding up the first open on cold
cache significantly.

Extension Manager: style fixes

Extension Manager: correctly handle component types

There were a few missing and they were also missing from the detail
list. Searching for components using the type: prefix now works.

Extension Manager: fix incorrect import

Extension Manager: check repo access in handle()

Still need to check if the behaviour on missing internet works okay.

Extension Manager: overhauled list display style

The GUI is still not fully functional again.

First start at refactoring the extension manager

The aim is to readd the existing functionality into proper classes, then
extend from there.