Add useful functions to GDriveUrl.

Summary:
I added the following useful functions in GDriveUrl:

• QString filename() const
• bool isTrashDir() const
• bool inTrash() const
• QUrl url() const
• GDriveUrl parent() const

Test Plan: Tests in UrlTest have been updated according to the new functionalities.

Reviewers: elvisangelaccio

Subscribers: KIO GDrive

Tags: KIO GDrive

Differential Revision: https://phabricator.kde.org/D20064