UI for setting scale range in date/time format

[cpascual]

The standard way of setting the scale range (via X/Y Axis submenu of the context menu) is not user-friendly when the axis is a DateAxisItem (e.g. the X axis in TaurusTrend widget) because the numerical values displayed and expected for the range limits are are "epoch" values.

A more user-friendly UI should be provided, ideally allowing to use both absolute and relative date strings, and possibly even using dedicated calendar/clock widgets.

As an inspiration, the UI in the old implementation in taurus.qwt5.TaurusPlotConfigDialog was quite friendly and compact (see, e.g., strtimetoepoch ).