Extend FunctionSourceCodeRequested().

Now takes an optional boolean parameter to indicate that disassembly is
explicitly being requested. Adjust TeamDebugger and LoadSourceCodeJob's
implementations accordingly.
Implement #9775.

- When possible, SourceView now adds a context menu option to switch
between source and disassembly. If the disassembled code is not yet
available, it is asynchronously requested. Adjusted SourceView::Listener
and implementing subclasses accordingly to make that request feasible.

- Adjust TeamWindow to correctly deal with the possibility of the
function source code being available but not loaded.