Feat: Average sensor value

[inckie]

Average sensor value calculation using basic continuous integration (not that one).
It's aligned to the current Min/Max values behavior, which is seems to be expected now (i.e. values are preserved since last app start/reset even if the original event is not in the timeline history anymore).

Known issues:

• calculates averages values for parameters that do not make physical sense (like accumulated totals for traffic, etc.). We can filter these out if needed.
• does not handle system time change in a nice way (plots will not handle it properly too, though).

Form was edited from code, since I can't open form editor (need to run IDE in x86 mode or something, probably).

Comments welcome.

#967

[inckie]

@PhyxionNL PR is ready for merge, any change request?