Remove insertIntoTextEdit() from Signature's public interface

It's not used externally, so we can easily do that.

The motivation behind this is to separate core class (like Signature) and
widget code that pulls in the full rich text editing stack. This will e.g.
help background processes such as the reminder daemon, which use identity
data, but that don't have any editing UI for it themselves.

There is a second piece missing for this still, findImageNames() in
signature.cpp.