Linux support

Added

• Now supports Linux — available via Flathub!
• New langauges: Norwegian (thanks to @comradekingu) and Czech
• Markdown notes for timers
• Notifications
• Setting for allowing a single timer at a time

Changed

• Updated dependencies
• Code cleaned up a bit

Fixed

• Various translation fixes, thanks to J. Lavoie, @liimee, and @wmfadel