Automate min_size

[sh-rn]

TBD:

Min_size needs to be abstracted from the UX, as it is strange and confusing for most traders.A standard min_size (10%) shall be employed.
Min_size should be exposed in advanced settings, to give power users control when they want it.

To avoid network fee and dust threshold issues, min_size shall automatically adjust upwards when order size falls below a certain threshold.

BTC and ETH require larger min_size than other coins.

Pending XBridge enhancements, this should be implemented in Block DX.