Add #include directive to .stignore (fixes #424)

Though breaks #502 in a way, as .stignore is not the only place where
stuff gets defined anymore.

Though it never was, as .stignore can be placed in each dir, but I think we
should phase that out in favor of globbing which means that we can then
have a single file, which means that we can have a UI for editing that.

Alternative would be as suggested to include a .stglobalignore which is then synced
as a normal file, but gets included by default.

Then when the UI would have two editors, a local ignore, and a global ignore.