ui: prune/gc view: improve sizing & scrolling behaviour

ui: prune/gc view: improve sizing & scrolling behaviour

before, this was only used where the top list was a fixed size and only
for one datastore (which limits the number of prune jobs a bit)

since now we show gc jobs for all datastores here too and all their
prune jobs, this panel can get much bigger.

To improve it's scrolling sizing behavior, make the prune jobs panel
`flex: 1`, so it fills out the rest of the view, and add a splitter
between them so one can resize them on the fly. To prevent making one of
the panels too small, set an appropriate minHeight for both and make the
surrounding panel scrollable.

To not save the height into it's state, we have to filter that out for
the GCView.

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>