ui: add MultiDiskPanel

ui: add MultiDiskPanel

this adds a new panel where a user can add multiple disks, intended
for use in the wizard.

Has a simple grid for displaying the already added disks and displays
a warning triangle if the disk is not valid.

this is a base panel for adding multiple disks/mps for vms/ct
respectively.

this combines the shared behavior and layout and defines the functions
that subclasses must define

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>
Tested-by: Lorenz Stechauner <l.stechauner@proxmox.com>
Tested-by: Aaron Lauterer <a.lauterer@proxmox.com>