Small GUI improvements

Selectable wallet address, resizable main window with a movable splitter on the exchange tab. Short video if you want to quickly see how it looks and behaves on my system: https://www.youtube.com/watch?v=rRvbGxyGlq0

For some reason layout alignment of the wallet address label works fine in Qt Creator but is ignored in the program, so the label is lower than it should be. Any idea why it happens?