Add separate options for window control buttons

[Niapoll]

Description

Previously, window_controls was an all-or-nothing toggle that rendered all three controls (minimize, maximize, close) in fixed geometries. On certain Window Managers (WM) and Desktop Environments (DE), minimizing windows is unsupported or handled entirely via keybindings, leaving non-functional buttons or unwanted controls in the top bar.

With this change applied, we'll have individual configuration options to enable or disable each window control button independently.

The control box width, element hitboxes, and button layouts are now calculated dynamically based on which buttons are active. Remaining controls neatly right-align without leaving dead space or misaligned click zones.

---

Previews

Default (All Buttons)	No Minimize
Only Close	Custom Combination

---

Testing

• Tested with various button combinations (all on, only close, min disabled, all disabled).
• Verified click hitboxes, hover effects, and title clipping dynamically adapt to the resized control box.
• Tested across windowed and fullscreen/borderless modes.

---

Changes

• modernz.lua: Added window_close_button, window_min_button, and window_max_button options and dynamic geometry calculation.
• modernz.conf: Added the new options to the default configuration template.
• docs/USER_OPTS.md: Updated the options table in the documentation.