cannot paste into text fields from mobile safari

Using the [demo](https://emilk.github.io/egui/index.html) on an iphone you cannot paste into text fields.

First observed in [tpscube](https://github.com/d0ntPanic/tpscube/) but appears to be an egui issue since it reproduces in the demo.

I realize this is potentially hard to work-around given the canvas backing, but one potential work-around is to optionally render a browser native text-field (and getting the exact style correct 😬)

Start by reproducing the issue in the linked egui demo on iPhone Safari, then trace the canvas-backed text-field input path. Check how paste events are handled and compare that with the suggested browser-native text-field workaround. Done means text can be pasted into egui text fields on mobile Safari without breaking existing editing behavior.

Written by the indexing model from the issue text.