Checklist

• I have searched the issue tracker for open issues that relate to the same feature, before opening a new one.
• This issue only relates to a single feature. I will open new issues for any other features.

Is your feature request related to a problem?

Currently, when tabbing through multiple fyne widget.Entry form fields, text of active entry will not be selected. In contrast to (most? all?) web browsers - they will select the content when tabbing from one <input type="text"> field to the next; users might be used to this behavior and it's probably more efficient.

HTML form example that displays said behavior:

<form>
  <input type="text"> 
  <input type="text" value="test 123">
  <input type="text">
  <textarea>Foo bar</textarea>
</form>

Is it possible to construct a solution with the existing API?

I don't think that it's currently possible, as there are no public Entry methods to e.g. SelectAllText().

Describe the solution you'd like to see.

It would possibly be a rather small change in the FocusGained() method:

	if !e.MultiLine {
		e.sel.selecting = true
		e.sel.selectCurrentRow(true)
	}

This would keep behavior consistent with browsers, who also do not select textarea contents on focus.
It seemingly doesn't interfere with e.g. mouse clicking into an Entry -- no selection happening.

If enabling this by default is not desired, having a bool toggle on the Entry widget would be great, too.