Improve Custom Field expression dialog

[philmills]

I'm using v 2019
1) If you have a mistake in your expression syntax, you get an error message as expected, but on clicking OK the custom field dialog dissapears and you have to start again from scratch.
The dialog should remain open so you can fix the syntax
2) The dialog is too small, making it impossible to see your entire expression
3) When editing your expression, hitting the Delete key will delete the entire expression, rather than just deleting a single character where your cursor is (normal windows behaviour). There is no way back from deleting, undo doesn't work, so you have to start again from scratch.
4) Make it possible to rename the custom field