Entity Toggling

[SPFabGerman]

Based upon the request from @japhir at #2, I implemented a very simple first approach of entity toggling. It seemed to work fine, but I did not really had the time to further test this, so there might still be some bugs.
I tried not to write a new function for this, but instead implemented this with the functionality currently available, reusing as much code as possible.

But I have found one bug: After disabling the Org-Appear Mode, the cursor behaves wierdly, when moving over previously decomposed entities. I don't really know what is causing this, since the composition seems to be reapplied correctly, so the bug is maybe not even related to Org-Mode, but instead to movement functions? Maybe a look at prettify-symbols-mode (which does more or less the same thing as Entity Toggling) will help to understand what is happening here.

[awth13]

Thank you, @SPFabGerman! Didn't expect you to jump in so quickly.

Not sure what causes this bug -- based on my testing, doesn't seem to be our abuse of font-lock-flush and font-lock-ensure to prevent autoedits -- but there is an easy fix I came up with. Just (goto-char start) when disabling an entity (you have to check that it's an entity and not another element). I can add this myself after merging but feel free to implement it.

Apart from that, I don't see any issues so far. One suggestion I have is to skip these lines entirely. pcase evaluates to nil when there is no match, and I think it's fine that entities won't have any visible start/end. Would you mind changing that before we merge?

[SPFabGerman]

Hey, thank you. The suggestion with goto-char worked seemlessly.
In order to remove the unused visible-start/end properties I added some conditional checks to handle entities seperatly. (Therefor I had to include the entity-type variable in the show and hide functions too.)

But while testing, I found another very wierd bug. When using Org-Appear Mode, but without Entity Toggling, the cursor moves wierdly over the Entites. I found that point is changed when moving, so it looks like Emacs tries to move over the "real" text and not the composed Entities, which it shouldn't do. But I think this issue might be unrelated to this Pull Request, since the same thing happens in the Master Branch. I don't know what is causing this, since Org-Appear shouldn't even mess with the Entites in that case.

[awth13]

Thank you, @SPFabGerman, great work!

Again, not sure why this behaviour happens, but I found how it happens. Because org-appear does not forget the previous element after it was hidden, it rehides it after each command until a new element is encountered. This, somehow, messes with movement. Fixed now.

You work here alerts me, in general, that org-appear affects movement more than it should. Thank you. It's unrelated to this PR, though, so I will do some more testing and merge.