Add a flag to hide the cursor in hyprpicker

[ItsProfessional]

Resolves hyprwm/contrib#92

I know that the flag itself seems unnecessary/unfitting for this project, but considering that a good chunk of users use hyprpicker solely for "freezing" their monitors, I see this as necessary.

Because, without this (in the case of grimblast), the user's actual mouse cursor isn't captured in their screenshots. Instead, a part of a crosshair cursor is shown in the corner of their screenshot (when taking a screenshot of a region).

Requested changes: When the flag is passed,

• Capture and show the user's original cursor (captured before running hyprpicker) as well in hyprpicker.
• Hide the user's current cursor (the crosshair shown when the user moves around their mouse in hyprpicker).

I'm open to create a PR if maintainers aren't opposed to this proposal.

[SoSeDiK]

Considering this was not a goal of a color picker utility, but it's still used as a workaround for freezing the screen, maybe this specific functionality (freeze screen & optionally hide cursor) could make it into a separate util, e.g. hyprfreeze, or even Hyprland's hyprctl dispatcher?
Would be quite useful for scripting. The only thing, I'd rather have it under hypr* umbrella, so anyone using that functionality wouldn't hesitate to switch.