[GUI] Small improvements to Relative Orders Configuration

[MarkoPaasila]

• The "Amount" input field should be followed by the quote asset name in question
• The "Center Price" input field should be appended with "[base]/[quote]"
• Move "Spread" input above "Center Price"
• Give "Manual Center Price Offset" a slider from -10% to +10% and default to 0% (it is a tool that should be very easy to use)
• Change "Amount" label to "Order size"
• Change "Calculate center price dynamically" to "Update from closest market orders"
• Remove dropdown for base asset selection and leave blank input field. (This can be done to all strategies)
• Add info text: "Asset to be used as unit of measure" after base asset input field
• Add info text: "Asset to be bought and sold" after quote asset input field