Add history cap in memory units

[azonenberg]

In addition to the user controlled history depth, we should also prevent the history from using too much RAM.

The actual history cap should be the minimum of the depth specified in the history window and a preference which specifies the max history depth, in GB.

Additionally, when adding new waveforms to the history, if total free system memory would drop below a safety margin (new preference), then old history should be purged regardless of preferences and the history depth cap.

[azonenberg]

Closing as wontfix since we now have the option of pushing history to files instead, relieving RAM pressure. Needs more internal work but that is the long term plan.