Update `NotifyIcon` references/centralize management code

[gbakeman]

The NotifyIcon is referenced throughout WinNUT.vb code and in other Forms that could make the state of it inconsistent.

• Make the NotifyIcon instance private - any references in external code need to be removed and logic integrated into the main WinNUT form code.
• Upgrade the UpdateIcon_NotifyIcon method in WinNUT.vb to handle all other cases throughout the class that affect the state of the NotifyIcon, and modify it accordingly. Also consider renaming this method to something like NotifyIcon_UpdateState