Reduce drawing flicker in SourceView.

Instead of relying on the app_server to handle the background color, do so
ourselves. This allows somewhat more granular control, and helps reduce
flicker on drawing when single stepping.
Reduce flicker in StackTrace display when single stepping.

When asked to clear the current stack trace, delay actually doing so
by .25 seconds. If the stack trace is set to a new one in the meantime,
the operation is aborted so we don't reset the trace twice.

Thanks Ingo for the suggestion.
BTabView: minor optimization.

Short circuit if asked to select the same index that's currently visible, to
avoid unnecessary invalidation/flicker.

Should fix last part of #9841.