Profiler: Show "truly-in-use" memory usage line

[manopapad]

This would handle a common pitfall for users when looking at profiles: Users see the memory usage only go up, and assume that the program is leaking memory. In reality the instances that are accumulating are probably invalid, and would be removed by Legion if it actually got squeezed for memory.

IMHO what the user truly wants to know is how much "breathing room" they have on the memory at any given time.

We would need the profiler to be told when instances become valid and invalid, so it can draw a line of the "truly-in-use" memory usage, which only includes valid instances (possibly in addition to the "technically correct" utilization that we show today, where invalid instances are also included).

Assigning @lightsighter to give an estimate on the work, maybe make sub-tasks to assign.