Dialogs are not refreshed correctly on .Show()

[Jacalz]

Describe the bug:

The dialogs are not correctly refreshed when calling .Show() on a dialog that has already been created. This can be indicated by saving a reference to a dialog, hiding that dialog and then either changing the theme or resizing the window causes it to have the wrong colour or being at the wrong screen position.

To Reproduce:

Steps to reproduce the behaviour:

1. Save a reference to a dialog
2. Open the dialog
3. Resize the window and/or change theme
4. Notice incorrect size, position and/or wrong theme

Screenshots:

Example code:

package main

import (
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/container"
	"fyne.io/fyne/v2/dialog"
	"fyne.io/fyne/v2/theme"
	"fyne.io/fyne/v2/widget"
)

func main() {
	a := app.New()
	w := a.NewWindow("Dialog resize")
	a.Settings().SetTheme(theme.LightTheme())

	themes := "light"
	ctheme := widget.NewButton("Change theme", func() {
		if themes == "dark" {
			a.Settings().SetTheme(theme.LightTheme())
			themes = "light"
			return
		}

		a.Settings().SetTheme(theme.DarkTheme())
		themes = "dark"
	})

	dialogs := dialog.NewInformation("Close me", "Close me and then resize window and/or change theme", w)

	w.SetContent(container.NewVBox(
		ctheme,
		widget.NewButton("Open dialog", func() {
			dialogs.Show()
		}),
	))

	w.ShowAndRun()
}

Device (please complete the following information):

• OS: Linux
• Version: 5.10.9
• Go version: 1.15.7
• Fyne version: 2.0.0

[andydotxyz]

On develop and release/v2.0.x branches