switch GUI x-axis to use actual time values instead of snapshot indices

_From [derek.br...@gmail.com](https://code.google.com/u/117968039472581148324/) on December 10, 2010 19:52:20_

PR 560868

today the GUI uses snapshot indices as x axis values.
with the peak snapshot (PR 476018), using snapshot indices presents a misleading picture as to the relative
time of the snapshots. also, today there is no way to find the actual time value of a snapshot.

this PR covers switching the x axis to use actual time values.
the slider should also be changed. it's fine for its granularity to remain regular snapshots: i.e., it's fine
if the peak cannot be isolated on its own and its surrounding regular snapshots must be shown.

to make it clear that there is no data between snapshots, either points should be shown,
and/or both the snapshot # and time value should be shown in the hover info.

_Original issue: http://code.google.com/p/drmemory/issues/detail?id=229_