[FEATURE] Place recently used currency pairs on top (RT-1876)

* Change to "recently used" instead of "recently traded".
* Add watch in rpCombobox in order to update list at once.

[BountySource](https://www.bountysource.com/issues/2842748-ripple-trade-place-recently-used-currency-pairs-on-the-top-of-the-drop-menu)
[RT-1876](https://ripplelabs.atlassian.net/browse/RT-1876)
Do we need this comment?