Implement backspace soft override

[planarvoid]

Fix

In this PR I'm adding an option to override the backspace on the soft keyboard. This is useful if you want to show a dialog instead of deleting a certain item (like an image).

In order to test it you have to:

• Create a placeholder manager instance and register it with Aztec
• Create ImageWithCaptionsAdapter and register it with the placeholder manager
• Change the EXAMPLE string to something like this:

      private val EXAMPLE = """
          <p>Line</p>
          <placeholder type="image_with_caption" src="https://file-examples.com/storage/fe1aa0c9d563ea1e4a1fd34/2017/10/file_example_JPG_100kB.jpg" caption="test"
          <p>LIne</p>
      """.trimIndent()

• Register the placeholder backspace listener with the aztec like this:

  .addBeforeBackSpaceListener(PlaceholderBackspaceListener(visualEditor) {
  true
  })

Test 1

1. Do the initial steps
2. Click right behind the placeholder image
3. Click on "Backspace"
4. Notice the image is not deleted

Test 2

1. Do the initial steps
2. Click at the beginning of the next line behind the image
3. Click on "Backspace"
4. Notice the image is not deleted

Test 3

1. Do the initial steps
2. Click anywhere that's not right behind the image
3. Try backspace
4. Notice the text is deleted as expected

Review

@khaykov

Make sure strings will be translated:

• [x] If there are new strings that have to be translated, I have added them to the client's strings.xml as a part of the integration PR.

[planarvoid]

Thanks for testing @khaykov ! I think both of these are kind of ok. If the user taps a lot of times, I think it's fine to just let it pass. We'll also show a dialog in the DO app so I think this will not happen IRL. This is a nice-to-have feature so I think it's also OK if it doesn't work on some devices

[khaykov]

Got it 👍 Then it's :shipit: