Add getprop dispatcher to complement setprop

[ninjack-dev]

Currently, the vast array of window properties usable with setprop are inaccessable to a user. The ability to retrieve this information in a similar way would be very much appreciated.

As a practical use case, hyprctl dispatch setprop activewindow alpha <some alpha> can be used to set the opacity of a window, but it is currently impossible to adjust the alpha with a delta, like would be done when adjusting audio volume or screen brightness. A getprop dispatcher could be used to trivially retrieve the current alpha for this purpose:

# This example doesn't account for clamping the value in case an opacity of 0.0 is undesired
hyprctl dispatch setprop \
	activewindow alpha $(echo "$(hyprctl dispatch getprop activewindow alpha) - 0.1" | bc) 2>/dev/null

The only current option (that I'm aware of at least) is an awkward implementation of tagging windows with the needed values and extracting them with complicated scripts.

[MightyPlaza]

Additionally the alpha struct could have a few cpp operators implemented to be able to be incremented like float, int or bool props (https://github.com/hyprwm/Hyprland/blob/main/src/desktop/WindowOverridableVar.hpp)

hyprctl dispatch setprop activewindow bordersize relative +2

[ninjack-dev]

This would be nice as well. I personally want to be able to access the existing state so that I can clamp the value; I never want an opacity of 0, generally speaking.

[MightyPlaza]

clamping is also implemented in #9566, but there is no current interface with the user

[ninjack-dev]

Oh, very nice. Can't wait to have that in place.

[vaxerski]

#11191