Improve the signature help

[jdneo]

Currently the implementation of SignatureHelp could provide paramter hints for normal cases. I found that sometimes, it's not working properly.

Failure case 1

System.out.println("{,},(,),");

In this case, the signature help won't hint anything.

By reading the source code I found that the root cause is that in the implementation of SignatureHelpHandler#getContextInfomation() is not robust. For example, it's not taking the appearance of ( and ) in a string literal into consideration.

Failure case 2

For project spring-petclinic:

While the actual signature is: SpringApplication.run(Class<?> primarySource, String... args)

That means the varargs is not handled properly.

Failure case 3

While the actual signature is: SpringApplication.run(Class<?> primarySource, String... args)

That means when the parameter list is not complete, we cannot handle it properly.

I'm now trying to fix those problems recently. Hopefully, I can send a PR in around next week. :)