Implementing autocompletable text editing

[crumblingstatue]

In #122 it is suggested to use popup_below_widget.
I tried that method, but there are several things that make it unergonomic for the user.

• Up/Down keys always change the cursor position in TextEdit, so the cursor keeps changing while the user is navigating the dropdown list.
• After the autocomplete happens, the cursor doesn't change position, so it's in the middle of the changed word instead of at the end.

If someone has implemented an autocompletable text edit widget in egui, I would love tips and suggestions.

[crumblingstatue]

I opened this pull request #777