class library: signal blend returns a signal. Fixes issue #1004
class library: add a more informative error message to scope

When the computer sleeps, the shm interface may get lost. Then the server, if not internal, can't scope. Instead it tries to use the internal stethoscope, which now throws a more informative error.
Note: the GUI redirects in this case are extremely intransparent and should be refactored.
class library: change from spaces to tabs in QT related files

Instead of spaces, tabs are standard in the sc class libraray. This change avoids unintended mixing of conventions.