ENH/FIX Allow plotting a selection of source spectrogram

* Calculate time-frequency source estimates with padding, i.e. from -0.6s to
0.8s, but display a spectrogram from -0.3s to 0.5s.
* Thus ensure that all time-frequency bins are calculated as averages of
multiple time windows (without this fix in the example the source power in
the first and last time bins was not an average, which could lead to
unexpectedly high o low activity at the edges)

This enables calculating spectrograms with padding
FIX Unnecessarily wide time range