Open clangd config files from prefererence/properties page

[ghentschke]

As a user I want to change the clangd settings via the .clangd on project or via the config.yaml file on user level as described here in a user friendly way.

Since the .clangd file is not visible to the user by default in project explorer, because of the filtering for .* files, the solution could be a open-config-file button in the editors preference/properties page.

When there is no such file, it should be created with a default content.