* lib/api, lib/connections, gui: Show connection error for disconnected devices (fixes#3345)
This adds functionality in the connetions service to track the last
error per address. That is in turn exposed in the /rest/system/status
API method, as that is also where we already show the listener status
from the connection service.
The GUI uses this info where it lists addresses, showing errors (if any)
in red underneath each address.
I also slightly refactored the existing status method on the connection
service to have a better name and return typed information.
* ok
* review
* formatting
* review
* cmd/syncthing, lib/gui: Separate gui into own package (ref #4085)
* fix tests
* Don't use main as interface name (make old go happy)
* gui->api
* don't leak state via locations and use in-tree config
* let api (un-)subscribe to config
* interface naming and exporting
* lib/ur
* fix tests and lib/foldersummary
* shorter URVersion and ur debug fix
* review
* model.JsonCompletion(FolderCompletion) -> FolderCompletion.Map()
* rename debug facility https -> api
* folder summaries in model
* disassociate unrelated constants
* fix merge fail
* missing id assignement
