Add option to set placeholder size based on the item shown

[planarvoid]

Fix

This PR adds an option to the placeholder sizing where you can define placeholder size based on the item (and the AztecAttributes). This is good for things like images and videos which might not always be in landscape/portrait and showing them differently might result in a non-perfect experience

Test

1. Set up MainActivity with the placeholder manager and the ImageWithCaptionAdapter. The steps are:
   • Create PlaceholderManager in the onCreate method in MainActivity passing in the visualEditor
   • Call placeholderManager.registerAdapter(ImageWithCaptionAdapter()) to register the ImageWithCaptionAdapter
   • Call aztec.addPlugin(placeholderManager)
   • Call aztec.addOnMediaDeletedListener(placeholderManager)
2. Modify the getWidth and getHeight methods in order to change them based on the item shown. For example:
   • Set EXAMPLE to <placeholder type=\"image_with_caption\" src=\"https://file-examples.com/storage/febc474733629f43d9f078c/2017/10/file_example_JPG_100kB.jpg\" caption=\"1\"><br><placeholder type=\"image_with_caption\" src=\"https://file-examples.com/storage/febc474733629f43d9f078c/2017/10/file_example_JPG_100kB.jpg\" caption=\"2\">
   • Modify getHeight method in ImageWithCaptionAdapter so that it returns different value based on the caption attribute`
3. Run the app
4. Notice the images are drawn differently

Review

@danilo04

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.