backup config: add icons to tab buttons

backup config: add icons to tab buttons

From a UX perspective flat buttons, i.e. buttons without a frame or
background, are not obviously clickable without an icon.

The choice of icons was taken from the storageSchema map in PVE.Utils
module currently residing in the pve-manager repo.

Signed-off-by: Maximiliano Sandoval <m.sandoval@proxmox.com>
 [TL: clarify where the choice come from and other small nits]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>