Add optional "id" element to manifest

[mrclay]

For various reasons (e.g. ​cloning a repo), one may end up with a plugin with the incorrect directory name inside mod, which can lead to serious problems (e.g. if the owner renames the folder after enabling!).

Adding an optional (for now at least) id element would allow us to help admins correct this error immediately.

[mrclay]

1.9 PR ​https://github.com/Elgg/Elgg/pull/267

[mrclay]

PR adds this behavior:

If manifest has an "id":

...then plugin ID must match. Otherwise:

[mrclay]

This is better than nothing, but I'd like a name mismatch to still be valid but canActivate() = false and to generate a red warning like the unmet dependencies. Unfortunately the warning code assumes that not being to activate implies it must be a deps problem, and I don't want to add a bunch more branching logic to the plugin full view.

[Steve Clay]

Fixes #4555: adds "id" to manifest and, if present, assert that directory name matches.
Also documents $plugin in some views to help IDEs with code comprehension

Changeset: 8a7e22be5a90e5be3902631f39fabab919d3e0b9