Correct order of themed plugin paths.

The themed plugin path should precede the generic theme path. This makes
it possible for a plugin to provide specific views for specific
controllers even when plugins contain controllers of the same name.

Refs #4064
Simplify view path generation.

Use fewer array_merge() calls so the code should theoretically be a bit
faster.

Refs #4064
Merge pull request #4081 from cakephp/issue-4064

Fix incorrect order of view paths for themed plugins