Support projects without build.gradle

[sdedic]

A multiproject build looks, according to the documentation like

├── app
│   ...
│   └── build.gradle
└── settings.gradle

there's no build.gradle in the root. While GradleFiles.isSubProject already works OK with build-less projects, the isRootProject does not. This PR fixes that. Simple test added.

During other work, I've realized that the ProjectFactory2 implementation for both Gradle and Maven do not return project type -- although they could. The display name is not that easy to evaluate, but both project types use a fixed ID, so it's simple to return.

[lkishalmi]

Sounds fine!

[lkishalmi]

Yes the project ID-s are not really used in NetBeans, a few Ant based projects uses that ID, but that's all.

[sdedic]

Yes; but for certain purposes it may be important to differentiate the build system, i.e. for some abstractions that map to different actions/goals/properties/whatever in each build system. It was already in the APIs, so I've just filled the gap.
It's a pity that displayName remains unfilled - the isProject2 could be a good query for e.g. file selector.