feat: add stayfocused window rule

[prankstr]

Add stayfocused window rule that forces focus on a window as long as it's visible, similar to Hyprlands stayfocused windowrule. Fixes #558

Useful for password managers and other popups where you don't want to lose focus by moving the mouse.

It seemed least invasive to re-use exlusive_focus since it already does half the work, so this PR:

• Adds stayfocused property to window rules
• Sets exclusive_focus when a stayfocused window gains focus
• Adds early return in focusclient() to prevent focus changes while a stayfocused window is visible
• Clears exclusive_focus when the stayfocused window is unmapped

Usage:
windowrule=stayfocused:1,title:1Password