Svg icons for move track up/down buttons on the MIDI import panel

Add svg arrows instead of captions "Up", "Down" for buttons on the MIDI import panel.

Light theme:
https://drive.google.com/file/d/0B5alKuFoSol2NVB4MXVGa3JRWHM/view?usp=sharing
Dark theme:
https://drive.google.com/file/d/0B5alKuFoSol2R0haWldLS2lxVEk/view?usp=sharing