Add get_entity to Message. (Fixes #400) - #404

Good work @bomjacob!

I think at this point it makes sense to add a get_entities function that takes as argument entity_type (string) and returns a list of all the matching entities, and when entity_type is None, it returns all of them. With that this PR is good to go.

Nice work @bomjacob and @rahiel!

I also like the idea @rahiel, and it would be best to add all possible values for MessageEntity.type as constants in MessageEntity too.

By the way, this PR is somewhat related to #362 because both add methods to Telegram classes.

Danm.. I wrote that before I saw what you wrote @jh0ker. I'll pull the possible values out and add them as a constant instead.
Also... this might be an issue (depending on what we want): text_link and text_mention has extra data than just some text, which currently wouldn't get returned by get_entities... Should we somehow add that?
And correct me if I'm wrong, but this shouldn't really affect #362, should it? I mean these methods kinda work as class methods anyways, right?

Was that what you had in mind in terms of constant @jh0ker ?

Actually I had something in mind like this or this.

On your other question, why not just return a list of MessageEntity instead of a list of strings?

Want them to be in the MessageEntity class, or in a seperate class?
And if in the same class, is there any easy way to get them all?
We could make it a subclass. So it would be fx. MessageEntity.Type.TEXT_MENTION.. but that might be annoying to write.

Yeah in the MessageEntity class is fine IMO. You can still have the list of all types, just add a constant for each of them too.

You renamed it to ALL_TYPES in messageentity.py but forgot to change it in message.py ;)

Forgot to push :P

Don't merge yet. I wanna quickly write a test.

Should this be changed?

Args:
     types (Optional[list]): Only extract text of MessageEntities if its type
     in in this list. Defaults to all types.

Like do I need to specify that it's a list of MessageEntity types somehow, or is this okay?

I think most docstrings have to be updated since they are referring to text instead of entities. Additionally, it might be a good idea to document when and why you should not use Message.entities directly and instead use these helper methods.

Also, maybe it would be a good idea to rename get_entity to get_entity_text, just to make clear what it does. And can't you re-use get_entity_text in get_entities? I think there is some duplicate code there.

While discussing with @bomjacob, we were re-thinking the MessageEntity.text attribute. Right now, it's not there by default and only "magically appears" if the entity object is processed by Message.get_entities.

Possible courses of action:

• Leave it like it is
• Define it with None as default
• Make it a property and raise a meaningful exception when queried before set
• Remove it completely and instead return a dict from get_entities that maps a MessageEntity to their text

Alright. It's been decited to go with: "Remove it completely and instead return a dict from get_entities that maps a MessageEntity to their text"
Latest commit should make the necessary changes and also rename the getters to something more appropriate.
Note: currently missing proper docstrings.. hold on

We might need an extra snippet at the end of https://github.com/python-telegram-bot/python-telegram-bot/wiki/Code-snippets for this.