Replace export pack mode "Copy" with "Exec+Zip"

Created by: leonkrause

Comment on this from #godotengine-devel:

16:04:38 <eska> what's "Pack mode: Copy" supposed to do? sounds like it's supposed to just recreate the file structure (instead of a pck), but it doesn't seem to work
16:11:06 <eska> reduzio, punto, seems like it's that way since 1.1, so probably no one knows other than you? ^
16:16:44 <reduzio> eska, that needs to be removed, it's pretty dangerous :|
16:17:14 <reduzio> godot support exporting all the files to a folder instead of packing them and use them unpacked
16:17:42 <reduzio> but updating a directory tree is kinda dangerous, so it's either changed to work exporting a zip with all stuff, or removed

This change removes pack mode "Copy" and adds "Exec+Zip". I also renamed the options to hopefully be more expressive. Additionally, bundle is now a separate toggle.

Related: #2788, #2879 (closed)