X11: Add support for custom window icon - #6475

I'm just getting into kitty and, while reading the FAQ, I fell in love with DinkDonk's "dark kitty" icon. Imagine my dismay when, after successfully setting it as the launch and taskbar icon, I discovered that a custom window icon is supported in macOS, but not X11!

This PR changes that. It uses the same logic (and filename) to locate a custom icon to use for X11. Specifically:

• For OS windows under X11, if a file named <config_dir>/kitty.app.png or <config_dir>/kitty.app-128.png exists (preferring the 128px image, per the note in the source¹), it will be set as the window icon. Otherwise, the 128px version of logo_png_file (<base_dir>/logo/kitty-128.png) is used.
• For notifications under X11, when icon=True, if a file named <config_dir>/kitty.app.png exists, it will be set as the notification icon. Otherwise, logo_png_file (<base_dir>/logo/kitty.png) is used.
• If no files named <config_dir>/kitty.app.png or <config_dir>/kitty.app-128.png exist, kitty's behavior under X11 remains unchanged.
• Under macOS, nothing should change (though some refactoring occurred), but I don't currently have access to a Mac to test this.
• Under Wayland, nothing should change, but I don't currently have access to a Wayland host, either.