Allow user to specify asset amount to buy or sell for XMR in range offer

[woodser]

Currently when taking an offer for a range, the user can specify how much XMR they want to buy or sell:

However, the user cannot specify how much of the asset they wish to buy or sell for XMR. For example, they cannot specify how much BTC to buy or sell for XMR in the screenshot above.

This issue requests being able to input the amount of asset to buy or sell for XMR, and auto-compute / adjust to the amount in XMR.