mixin/CBind: allow binding sub elements

mixin/CBind: allow binding sub elements

allow to access sub elements of configuration items for binding.

E.g., if the following configuration is present:

foo: {
    bar: true
}

one could do:

xtype: 'displayfield',
cbind {
    value: '{foo.bar}'
}
[...]

This mirrors the abillity of ExtJS's 'bind', which can do this also.
Simple implementation mirroring the surrounding code.

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