Look for directory to load in case a solution is opened

[iboB]

Currently the loaded directory in case a solution is opened is the directory of the .sln file. This can be... well... useless if the .sln file is not in the root of the repo, which is a popular pattern. Especially popular for externally configured CMake projects for example.

A smarter way to look for the root needs to be employed:

Use the first match of: (in that order)

• VSOpenFileFromDirFilters.json in parent directories of the chosen one
• .git subdirectory in the parent directories
• Choose a source file from one of the projects and repeat above two steps (will help with out-of source configurations)
• If all else fails, fall back to the directory of the solution

Some way of manually setting the directory should probably be available?