Restore "allow parens in applab image names"

[davidsbailey]

Reverts #20382, restoring #20368 , with fixes for images starting with http(s)://

changes made after reverting:

1. only escape relative filenames (those uploaded by the user)
2. change existing integration test to cover relative urls
3. add integration test to cover fully qualified url (starting with http://)
4. add integration test to cover absolute url (starting with /)

manual tested (relative path, absolute path, fully qualified url) in each of the following situations:

• design mode: screen, button, image
• code mode image(id, url):
• code mode setProperty(id, 'image', url):
  • screen
  • code-created image
  • code-created button
  • design-created image
  • design-created button

[islemaster]

👍 Yay additional tests.