Add AppleEvents usage description to manifest

[shayneholmes]

MacOS Mojave requires that applications provide the NSAppleEventsUsageDescription key in their manifest to communicate with other applications via AppleEvents.

This change provides the key, populated with a string that shows in an OS prompt when AppleEvents communication takes place:

I've taken a crack at wording the description, but the fix only depends on there being something there.

Fixes #821.

[ychin]

Quoting from #821:

Good question! Did some digging and found this:

MacVim, when compiled with the odbeditor option (:h odbeditor has more), implements the ODB Editor Suite, which is a protocol for external editors based on Apple Events. nvALT uses that protocol to leverage MacVim as an external editor. After launch, MacVim uses Apple Events to notify nvALT when the file is modified or closed.

Thanks! Yeah that makes sense.

For the wording, can you reword it slightly? I'm thinking something like "MacVim uses Apple Events to allow you to communicate with other apps using Apple Script, and to support working as an ODB external editor".

These "ask for permission" texts are usually designed to allow the app to justify itself why it needs that permission, rather that stating again what the permission is to the user. I think the only times MacVim would send Apple Events should be if it's an ODB external editor (this case), or the user uses Apple Scripts manually (either using :!osascript or Vim 8 terminal). It doesn't matter much, but might as well try to word it more accurately.

[shayneholmes]

Done. I took your wording verbatim, since it does a good job explaining how the permission will be used.