Optimize view paths caching for plugins.

Closes #2047
Merge pull request #3356 from ADmad/2.5-view-paths

Optimize view paths caching for plugins.
Allow setting only default layout without specifying template in email config.

Closes #3336
Merge pull request #3373 from ADmad/2.5-cakeemail

Allow setting only default layout without specifying template in email c...
update composer.json
Merge pull request #3376 from php-engineer/master-update-composer

update composer.json
Update docs for file().

Mention that relative paths will be prepended with APP.

Refs #3370
Reject file paths containing `..`.

Paths containing `..` are generally up to no good. Throw an exception,
as developers can use realpath() if they really need to get relative
paths.

Fixes #3370
Merge branch 'master' into 2.5
Merge branch '2.5' into 2.6