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

po/POTFILES.in

@@ -19,6 +19,7 @@ crates/pdfs-gui/src/app/pages/status.rs
 crates/pdfs-gui/src/app/pages/takeout.rs
 crates/pdfs-gui/src/app/pages/trash.rs
 crates/pdfs-gui/src/app/pages/verify.rs
+crates/pdfs-gui/src/app/update.rs
 crates/pdfs-gui/src/app/widgets/compat.rs
 crates/pdfs-gui/src/app/widgets/details.rs
 crates/pdfs-gui/src/app/widgets/file_list.rs