Add method to remove placeholder programatically

[planarvoid]

Fix

This PR introduces a method to remove a placeholder from the AztecText programatically.

• It locates the placeholder span based on predicate
• It removes the placeholder span
• it removes the wrapping media clickable span
• it removes the magic whitespace char that represents the span
• it refreshes the edit text

Test

1. Initialize PlaceholderManager in the MainActivity.onCreate method
2. Set aztec.addOnMediaDeletedListener(placeholderManager)
3. Set aztec.addPlugin(placeholderManager)
4. Register adapter placeholderManager.registerAdapter(ImageWithCaptionAdapter())
5. 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\">"
6. Run the app
7. Call the removeItem to remove a placeholder based on a predicate, for example I've tested it by adding the following call to the gallery button click:

placeholderManager.removeItem {
                    it.getValue("caption") == "1"
                }

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.