Improve consistency of opening windows and dialog boxes when working with multiple monitors

**What's wrong?**
When working with multiple monitors on a Mac (don't know how this is in other OSs), widget dialog boxes do not always appear where it is the most convenient.
I would prefer dialog boxes to open on the same monitor where the canvas resides that they belong to at that moment, with the possibility to drag it to another screen to create more space. The Options>Add-ons dialox box does work that way

**How can we reproduce the problem?**

Behavior in the case of two monitors:
When starting Orange, a canvas is always opened on the same monitor where the last canvas of the previous session was closed. Newly opened widget dialog boxes (and also Show Report and perhaps others) always open on that same monitor, even if the canvas was moved to the other monitor. Dragging a dialog box to the other monitor and then closing it, will cause it to open on that other monitor the next time, but the new opening location applies to that dialog box only.

**What's your environment?**

- Operating system: Mac OS 13.4 on Silicon
- Orange version: 3.35.0
- How you installed Orange: From DMG, updated from Add-ons menu

Start by reproducing the two-monitor behavior on macOS 13.4 with Orange 3.35.0, comparing widget dialogs and Show Report with the Options > Add-ons dialog. Trace how the canvas monitor and each dialog's saved position are selected. Done means dialogs open on the monitor containing their associated canvas while remaining draggable to another screen.

Written by the indexing model from the issue text.