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

packaging/aur/proton-drive-for-linux/PKGBUILD

@@ -78,4 +78,7 @@ package() {
 
   install -Dm644 LICENSE "$pkgdir/usr/share/licenses/$pkgname/LICENSE"
   install -Dm644 README.md "$pkgdir/usr/share/doc/$pkgname/README.md"
+
+  # pacman brings new releases, so the app never checks for them itself.
+  install -Dm644 /dev/null "$pkgdir/usr/share/proton-drive-linux/managed-updates"
 }