Organize existing code for editor plugins

[aaronfranke]

I was annoyed that the base class for editor plugins wasn't in the editor/plugins/ folder, nor were the files for changing editor plugin settings or editing project plugins. This PR moves these files into this folder.

Aside from moving files, this PR also modifies PluginConfigDialog to have the logic for selecting a language and script name organized together, instead of being mixed with the name/folder/description/author/version logic. This was the main motivation for this PR, I already have another PR lined up which needs this code nicely organized.

This PR has one behavior change: In the current master, when creating a new plugin, it will create a new script. However, if you change the file name of a plugin's script, it will not create a new one with the new name. Now it does. Note that it will only create a file if the file is missing, so it will never overwrite existing files.

[KoBeWi]

if you change the file name of a plugin's script, it will not create a new one with the new name. Now it does.

I think it should first try to rename/move existing file.

[KoBeWi]

Suggested change

	ERR_FAIL_COND(language == nullptr);
	ERR_FAIL_NULL(language);

[aaronfranke]

@KoBeWi I don't know the correct way to rename a script in the editor. If I just rename it using DirAccess, the script editor is not informed of the change, so the edited script has the wrong file name still.

[KoBeWi]

You can call update_file() in EditorFileSystem, or in worst case, just do rescan().

[aaronfranke]

@KoBeWi That does not inform the script editor of the change.

[KoBeWi]

And if you use this?
KoBeWi@7c165a1
(FileSystemDock::get_singleton()->rename_file())

[aaronfranke]

@KoBeWi That still does not work, and it gives a new error too:

[KoBeWi]

I was going to look into it, but mixing the behavior change with such enormous refactor makes it unreadable. You should split this into 2 commit.

[aaronfranke]

@KoBeWi I updated the PR to remove the behavior change. The other commit can be done in another PR.

[KoBeWi]

Plugin config dialog changes look fine.

[akien-mga]

Thanks!