This adds a "proportional state set" value to ProcessCore::Process, that is then
exposed as "Total Memory" column in KSysGuard. This column is hidden by default.
PSS is the amount of memory a process uses privately + the amount of shared memory
divided by the amount of processes sharing that memory. It is, as far as I know, the
closest we can get to "total physical memory usage" for a process.
The value for PSS is read from /proc/{pid}/smaps_rollup . This unfortunately means
we cannot currently display PSS for processes not owned by the current user, as
smaps_rollup is not world-readable.
For some background on this, see https://lkml.org/lkml/2017/8/9/1073 , which is the
patch that adds smaps_rollup and talks about what Android uses it for.