feat: "Main display" option for bar mode monitor setting #24

Before filing

• I searched existing issues for a duplicate.

Problem

When switching between laptop display + monitor and laptop display only, openmeters likes to stick itself to the laptop display, even when the monitor is attached and the primary display.

Proposed behavior

Add "Main display" option to for bar mode monitor setting. I suppose this could correspond to https://docs.rs/iced_layershell/latest/iced_layershell/reexport/enum.OutputOption.html#variant.Active?

Additional context

No response