Improve Asset balance view

Currently if an asset is in several open orders, there is no way to see how much of that asset belongs to an account.
All one can see if a bunch of orders, but not total amount.
Please add a view where one can see how much of an asset he has, regardless if it is in open orders.

Start by locating the existing asset-balance and open-order views in the BitShares UI and trace how account holdings are displayed. Define the combined asset amount across open orders, then verify that the new view makes an account's total asset balance visible regardless of order placement.

Written by the indexing model from the issue text.