feat(gui,core): opt-in check for new releases

Preferences gets a switch that asks GitHub once a day for the latest
release and shows a banner over the sidebar when it is newer. Off by
default. Packages whose package manager brings updates install the
marker share/proton-drive-linux/managed-updates, which hides the switch.

Refs #42

docs/CONFIGURATION.md

@@ -61,6 +61,7 @@ fix it.
   "proton_theme": true,
   "online_map": false,
   "tray_hidden": false,
+  "update_check": false,
   "files_view": { "list": true, "sort": "modified", "descending": true, "folders_first": true },
   "window": { "width": 980, "height": 680, "maximized": false },
   "open_with": {
@@ -87,6 +88,9 @@ fix it.
 | `proton_theme` | bool | follow system | General → Appearance | Proton's colours instead of the system theme |
 | `online_map` | bool | `false` | General → Appearance | Street map for Places from OpenFreeMap |
 | `tray_hidden` | bool | `false` | General → Appearance | Hide the tray icon |
+| `update_check` | bool | `false` | General → Updates | Ask GitHub once a day for a newer release and show it over the sidebar. Not offered where a package manager installs updates |
+| `update_last_checked` | Unix seconds | none | — | Written by the update check, so a restart does not ask again within a day |
+| `update_latest` | version | none | — | The latest release the update check found |
 | `files_view` | object | grid, by name | My files view menu | `list`, `sort` (`name`, `size`, `modified`), `descending`, `folders_first` |
 | `window` | object | 980 × 680 | Resizing the app window | `width`, `height` (the unmaximized size), `maximized` |
 | `open_with` | object | `xdg-open` | — | See [`open_with`](#open_with) |