Add support for copying and pasting to the X11/Wayland PRIMARY clipboard

**Is your feature request related to a problem? Please describe.**

In most applications using X11/Wayland, it is possible to utilize the primary clipboard by selecting text and pasting this selection using the middle mouse button, but not in egui. Because I want to make an application using egui that I will use myself (a level editor), this would add functionality that I expect from every application and would aid me in the usage of this tool.

**Describe the solution you'd like**

This crate uses `arboard`, which has primary clipboard support. It would not be hard to copy selected text into the PRIMARY clipboard, and paste it using a middle mouse button event.

Start by locating egui's clipboard integration through the arboard crate and its middle-mouse pointer-event handling for native X11/Wayland applications. Verify how selected text is copied to the PRIMARY clipboard and how a middle-button paste would be triggered; done means both operations work as requested without breaking existing clipboard behavior.

Written by the indexing model from the issue text.