This fixes an issue in SystemMonitor where old data would linger in the
table views after selecting a process owned by another user.
Since we can no longer read /proc/PID/* unless PID belongs to us,
we will now present empty views for these processes. :^)
Add dedicated internal types for Int64 and UnsignedInt64. This makes it
a bit more straightforward to work with 64-bit numbers (instead of just
implicitly storing them as doubles.)
Change the custom data massaging callback to take a const JsonObject&.
This will allow binding together data from multiple fields into one
output in the model. :^)
