With this commit now Vdebug places a stack sign (#>) on both Stack and
Source windows, to make clear which file and line it's loading from user
interface activity on Stack window.

The refresh method on Runner clears both signs.

Now Vdebug can query any stack depth, and the corresponding variables
and values will appear in the watch window. Also, the stack depth file
and line number will also appear correctly.

A visual sign (#>) was introduced to mark clearly the source and stack
windows for the current stack depth and file_and_line. When the
debugging resumes, these stack signs are removed from the ui.

Also updated the docs with a short description of these features.

A bug in previous commit was not setting runner.context_stack_depth to
zero when resuming the debugging session with a step or run command.

Also the Watch window was not showing the current file and line number
in the situation above.

Now a property expansion in Watch window list all children from that
property. The previous behavior was listing only <pagesize> elements
(i.e. based on the pagesize attribute of parent element).

As before, to invoke a property expansion either double-click it or
place the cursor over it and hit Enter.

The previous commit introduced the feature of getting all children from
parent elements in the watchwin tree.

This worked for property get UI commands issued from the Watch window
itself. Now, the default context get (when hit <F11> or stepping through
code or coming from Stack window) shows all children of tree depth==1.

Before this commit, context get would only show <pagesize> number of
children.