Bug 270500

Story:
1. I have already some namespaced classes in project root, let namespace be Acme
2. I create folder, let's name it Helpers
3. I create right click on this new folder, and choose "New" > "PHP Class", as I wan't to create class Acme\Helpers\StoreLoader, according to PSR-4 namespace rules.
4. Namespace input is empty, which is correct, as there are no classes yet
5. I click arrow of "namespace" input, to check if there are other namespaces

Current behavior:
6. The list is empty

Proposed behavior:
6. Fill list with namespaces from parent folders
6.a Have on top of the list, or directly in input: "Parent\Namespace" + "\CurrentFolderName"

Why?
Will be easier and less error prone to just add last namespace part "Helpers", or choose guessed one.
Folder to namespace mapping is compatible with PHP's psr-4 and psr-0 namespace/autoloader standard.