tree: prepare for ExtJS 6.2.0

tree: prepare for ExtJS 6.2.0

Remove the expand/collapse hack needed for an earlier ExtJS version.

Further do not collapse the rootnode before restoring the state, it
may not be collapsible at this time, as it may be still seen as leaf
node, and thus throws exceptions with ExtJS 6.2.0

we fired the refresh to late for the store, i moved it up to fire
directly after we resume events on the store, i.e. when our store update
window is closed again. So it happens before applying the state and
expanding the rootnode.

see
https://pve.proxmox.com/pipermail/pve-devel/2018-February/030762.html

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>