[Enhancement] When automatic updates are enabled, allow completely hiding the tray icon

Is your feature request related to a problem? Please describe.
Currently, if you have automatic updates enabled, the tray icon will still sometimes appear when it has found updates but they've not yet been installed.

Describe the solution you'd like
When automatic updates are installed, allow the option to always hide mintUpdate's tray icon.

Describe alternatives you've considered
One alternative I had considered is to just always immediately start installing updates right after it has checked for updates, but I believe the reason it checked but didn't immediately install is because there was a Firefox update but I was in the middle of using Firefox, so this alternative may not be valid.

Currently the only "solution" it seems is to straight-up disable Refresh the list of updates automatically checking for updates in mintUpdate, yet the automatic updater apparently is still able to find and install updates.

Additional context
There is a forum thread from around a year ago of a user with the exact same scenario as me (we both even have snapshots and the like configured):

• https://forums.linuxmint.com/viewtopic.php?f=47&t=350596

Start by tracing mintUpdate's automatic update flow and the code that decides when the tray icon appears. Compare the behavior described in the issue, including updates deferred while Firefox is in use, with the requested always-hidden option. Done means automatic updates can remain enabled while the tray icon is consistently hidden by user choice.