mintUpdate.py: Improve startup, process singleton behavior.

- Use Gio.Application to enforce uniqueness.
- Don't re-launch when displaying updates from a notification.
- Use os.execv() to restart mintupdate after self-updates.

Fixes #997.

mintUpdate.py: Improve startup, process singleton behavior.

- Use Gio.Application to enforce uniqueness.
- Don't re-launch when displaying updates from a notification.
- Use os.execv() to restart mintupdate after self-updates.

Fixes #997.

7.1.1

6.1.0

6.0.5

Tagging version 3.8.2