Change the behavior of Qt.core.resourceSourceBase

Currently this property is required and set to the product source dir by default. It works pretty good when somebody wants to group resources located in a fixed place in the project, especially in the product source dir. However it becomes inconvenient when a single resource should be put into qrc file with the specific prefix. Also it becomes super inconvenient when a single generated (output artifact) resource should be put into qrc because it's hard to say where in the file system it could appear (in the buildDirecotry or in its subdirectory which name is based on a hash from something).
To solve these problems I suggest:
1) Allow undefined value for resourceSourceBase. In case of this value qrc generator uses file name (without path) as alias for a resource and places it under the specified prefix ("/" by default).
2) Make undefined the default value of resourceSourceBase. This is required to prevent explicit binding of undefined value what looks a bit weird.