Hint user to install Language Support for Java if its not installed

[jdneo]

Most of the features in this extensions requires Language Support for Java to be installed.

We should hint to install it in the welcome view to make sure full features can be provided.

[jdneo]

To verify this (only used for test):

1. If Language Support for Java is not installed. hint user to install
2. If it's in LightWeight mode, hint user to import project
3. If workspace contains no Java projects, hint user to create one or open a project folder
4. Otherwise, show the project view as normal