New window control overhaul

Behaviour for someone with multiple microscopes to confirm:

• before: closing main window closes all windows. after: closing main window with other windows open minimises to tray
• before: #35 (closed) switching between microscopes keeps the same window open, but changes its target. after: one window per IP address
• before: connect never minimises to tray. after: closing main window when no other connect windows open closes the app fully

Closes #35 (closed) #34 (closed)