examples/keyboard_events Holding A and left clicking Clear button generates both Pressed and Held text

**Describe the bug**

**To Reproduce**
Steps to reproduce the behavior:
1. `cd tmp && git clone https://github.com/emilk/egui.git`
2. `cd egui`
3. `cargo run -p keyboard_events`
4. now hold `A` key and then left click on the `Clear` button once.
5. keep holding `A` key and left click on `Clear` as many times as you want.

**Expected behavior**

only `Held` should be seen after doing step 4 or 5.

**Screenshots**


**Desktop (please complete the following information):**
- OS: Gentoo Linux x64
- Browser n/a
- Version 2.13

**Smartphone (please complete the following information):**
- Device: n/a
- OS:
- Browser
- Version

**Additional context**

X.Org X Server 1.21.1.7

Run `cargo run -p keyboard_events` and reproduce the behavior by holding A while clicking Clear. Trace the keyboard and button event handling used by the `examples/keyboard_events` package, verify the issue against the latest master, and consider it done when repeated clicks while A is held show only `Held`.

Written by the indexing model from the issue text.