Add initial USB DDR pad support to the GUI

[rbs-jacob]

• [x] I have reviewed the OFRAK contributor guide and attest that this pull request is in accordance with it.

One sentence summary of this PR (This should go in the CHANGELOG!)

Add DDR pad support to the OFRAK GUI. Only maps buttons on the pad to keyboard shortcut callbacks – some more work is required to handle combos.

Anyone you think should look at this, specifically?

@EdwardLarson