[positioner] Tray positioning

[truchi]

Hello,

There is 3 issues I want to mention here with the positioner plugin's move_window(Position::Tray*):

• Panics before first tray event. Easy fix without API changes. Also seen in #437, #5, #173.
• Had a hard time figuring out that I need Position::TrayBottom* when tray is on top of the screen. A little documentation could have saved a lot of time.
• The window placement could be shifted to the sides to prevent it from overflowing beyond the screen's edges.

MacOS: 13.4 / 14.1 tauri: 1.5.2 tauri-plugin-positioner: 1.0.4

Thank you