Add a cache monitor to the System Load Viewer applet

Add a cache monitor to the System Load Viewer applet using 'cache/dirty'
and 'cache/writeback' infos from ksysguard 5.9.0

The monitor displays the dirty + writeback amount proportionaly scaled
with the maximal amount reached, until the remaining dirty amount goes
below a minimal threshold of 10MB.

An interesting usecase is to show the real progress of a datas transfer
to a slow storage device like a USB drive.

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