Add `anything` subject option to grimshot

[RagnarGrootKoerkamp]

This enables selecting a window, output, or area within one invocation, removing the need for multiple keybindings.

• Drag for an area (note that this already works in the window case)
• Click on a window to select it
• Click outside of all windows (e.g. in gaps or on the waybar) to select the entire output

Ideally this needs some testing with multiple monitors, which I don't have.

[OctopusET]

Would you squash your commits?

[RagnarGrootKoerkamp]

Oh, I should probably remove the mention that window also allows click-and-drag after #10.

Done.