Fix media removal and improve coroutine handling

[planarvoid]

Fix

The media removal is not currently added to history. That's fixed in this PR. Also the logic to remove media span is moved to the AztecText so that it's reusable. I've also fixed the coroutine usage and made the PlaceholderManager a coroutine scope.

Test

1. Set up PlaceholderManager in the main activity with the ImageWithCaptionAdapter
2. Make the EXAMPLE contain a placeholder that you can identify
3. Add removeItem action to something you can control (for example the camera button)
4. In the removeItem identify the placeholder you want to remove
5. Run the app
6. Remove the placeholder
7. Notice it gets removed correctly

Review

@danilo04

Make sure strings will be translated:

• 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.

[danilo04]

Nice work @planarvoid! Tested the changes and they are working properly !