ui: pci/usb mapping: rework mapping panel for better user experience

ui: pci/usb mapping: rework mapping panel for better user experience

by removing the confusing buttons in the toolbar and adding them as
actions in an actioncolumn. There a only relevant actions are visible
and get a more expressive tooltip

with this, we now differentiate between 4 modes of the edit window:
* create a new mapping altogether
  - shows all fields
* edit existing mapping on top level
  - show only 'global' fields (comment, mdev), so no mappings
* add new host mapping
  - shows nodeselector, mapping (and mdev, but disabled)
    (informational only)
* edit existing host mapping
  - show selected node (displayfield) mdev and mappings, but only
    mappings are editable

we have to split the nodeselector into two fields, since the disabling
cbind does not pass through to the editconfig (and thus makes the form
invalid if we try that)

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>