Gracefully handle multi-monitor systems

**Is your feature request related to a problem? Please describe.**
On multi-monitor systems, if the system tray is moved from one monitor to another (and that monitor has a different resolution), CopyQ's main window will get moved. Half the time, this is a good thing, because otherwise the CopyQ window may not be visible.

But the problem arises when the user moves the system tray back to the original monitor. At this point, the CopyQ window will be located at the wrong position. To fix this, the user can use a third-party tool to automatically move the CopyQ window, but alas, CopyQ will not remember this position when "copyq.exe exit" is issued (which is sometimes desirable due to #1249).

**Describe the solution you'd like**
Ideally, I would like CopyQ to remember its window's position ***relative to the nearest corner of the monitor*** (instead of using absolute monitor coordinates).

For example, let's say you keep the CopyQ window at 100px above and to the left of the lower-right corner of the monitor. All is good as you use your 2560x1080px monitor (for example) as your primary monitor. But now, let's say, you switch to a laptop's monitor that maxes out at 1280x720px resolution. No problem, the CopyQ window will move to a similar location on the smaller screen. But when switching back to the 2560x1080px monitor as primary, the CopyQ window will now be located near the center of the screen. Uh oh!

**Describe alternatives you've considered**
Well, long story short, I tried several alternatives, and I didn't like their limitations, so I decided to write this issue report.