SchemaShell always uses --file param when present.

Addresses **part** of the problem described in #4174.

Corrects SchemaShell's startup logic so that when a `--file` param is provided by the user, it is always used even in the case that it matches the default file name (schema.php). This solves a problem when a user wishes to provide a classname using `--name`, but still wants to use the default filename.
More predictable default for CakeShema class naming.

Instead of using the `APP_DIR` constant, which may change between developer installations of a project and cause issues loading Schemas generated elsewhere, use a fixed string, 'App'.

This is related to CakeDC/migrations#184 and should fully resolve #4174.
Figures it'd be a missing paren.
Corrects logic to only apply default filename in **all** cases where none provided.
CakeSchema naming fallback.

Abstracts the `require_once` of the schema file so it can be done twice. The added second call is a fallback for the previous APP_DIR-based naming to provide backwards compatibility.

Removes now-obsolete CakeSchema tests that involved `Configure::read('App.dir')`. The CakeSchema::name is now always static (and predictable) in the default case.
Code sniff fix.
Doc block capitalization corrections as requested.
Merge pull request #4230 from beporter/bug/schemashell-name-file-params-fix

SchemaShell --name and --file params priority fix.

Fixes #4174