Add pre-build commands.

[tadashuai]

trafficstars

Describe the issue (if no issue has been made)

1. Everytime after updating C# scripts, we need to build its solution manually, like Ctrl + B. Now just build Hazal.sln or HIT F5 directly!
2. Also see #582 .

PR impact (Make sure to add closing keywords)

List of related issues/PRs this will solve:

Impact	Issue/PR
Issues this solves	#582
Other PRs this solves	None

Proposed fix (Make sure you've read on how to contribute to Hazel)

1. Will build C# projects first when building Hazel C++ projects. Added the pre-build commands in the Hazel premake file. By the way, it checks whether the sandbox solution exists and notifies.
2. Only set C# project root namespace. <= Added namespace line in the C# premake files.

[tadashuai]

But if we only changed the C# code, it means the Hazel project doesn't need to be rebuilt and no pre-build commands are executed. At this point we still need to manually build the C# project.