Merge bitcoin-core#600: refactor: Add OptionsModel getOption/setOption methods

a63b60f refactor: Add OptionsModel getOption/setOption methods (Ryan Ofsky)

Pull request description:

  This is a trivial change which is needed as part of bitcoin-core#602 to get bitcoind and bitcoin-qt to use the same settings instead of different settings. It is split off from bitcoin-core#602 because it causes a lot of rebase conflicts (any time there is a GUI options change).

  This PR is very small and easy to review ignoring whitespace: https://github.com/bitcoin-core/gui/pull/600/files?w=1

ACKs for top commit:
  vasild:
    ACK a63b60f
  furszy:
    Code review ACK a63b60f
  promag:
    Code review ACK a63b60f.

Tree-SHA512: 1d99a1ce435b4055c1a38d2490702cf5b89bacc7d168c9968a60550bfd9f6dace649d5e98699de68d6305f02d5d1e3eb7e177ab578b98b996dd873b1df0ed236

Author: hebasto