The "Step Out" action continues execution until the current function
returns.
Also, LibDebug/StackFrameUtils was introduced to eliminate the
duplication of stack frame inspection logic between the "Step Out"
action and the BacktraceModel.
We can now step into library code in the debugger.
Since we now need the whole source code of our libraries
(and not just the headers), we clone the whole serenity git repo into
/usr/share/serenity.