Rename files with subclasses of PythonSignalState

[BCSharp]

These two files contain other parts of partial class PythonSignal (implementing module signal) so they belong together with file signal.cs. With this rename they are sorted together with signal.cs when displayed in the file explorer.

This commit is factored out from a larger upcoming PR for signal, but for the sake of a clean git history of renames I prefer to keep it as a separate commit, rather than squashed together with other changes.

[slozier]

Works for me. We could also consider preemptively adding the _ prefix _signal.NtSignalState.cs since in 3.5 the C module was renamed to _signal.

[BCSharp]

We could also consider preemptively adding the _ prefix _signal.NtSignalState.cs since in 3.5 the C module was renamed to _signal.

Not to defeat the purpose of this PR, this would require of renaming the existing signal.cs to _signal.cs as well. Unless you foresee problems with merges into 3.6, I'd keep the naming as is. I am counting on an assumption that with explicit renames like this one, merges should be easier as they should be able to track the file history across renames with 100% confidence.

[BCSharp]

To clarify: after the next merge to 3.6, rename signal.*.cs to _signal.*.cs on the 3.6 branch only in a separate commit.