rework panel/LogView

rework panel/LogView

use the view, viewmodel, controller style code, which makes the logic
much more understandable

this adds two features to it, namely a viewBuffer range, which controls
the distance to the end of the data on which the panel loads the next data
and an optional failCallback, which will be used for the cephlog

this also fixes #2151

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