Add -display SDL grab-on-hover option

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1884302

I'm using QEMU 5.0.0 on a Gentoo Linux host system. Guest is an Arch Linux
system.

I'd like to disable automatic mouse grabbing when the QEMU window is
focused.
I would prefer for QEMU to grab the mouse only after a click.

I use the i3 window manager on my host system.
Suppose I'm in workspace 1, while the QEMU window is in workspace 2.
In order to switch to workspace 2, I need to press the "Win+2" key
combination ("Win" is the Windows key).
The problem is that the character "2" (from "Win+2") will get transferred
to the guest system.
For example, if I have a text editor opened under the guest system, the
character "2" will be pasted inside the document I'm working on, which is
pretty annoying.

I would like instead to press the "Win+2" key combination and then
explicitely click on the QEMU window with the mouse before grabbing it.

Command line:

qemu-system-x86_64 -drive
file=/home/fturco/qemu/arch.img,media=disk,index=0,if=virtio,format=raw,cache=none
-cpu host -m 2G -k it -enable-kvm -net nic,model=virtio -net user -vga
virtio -display sdl -usb -rtc base=utc -soundhw ac97 -monitor stdio -no-
quit