MAIN: Encapsulate SoundControls more
MAIN: Rename SoundControls to PanelPreviewSound
GUI: Move GUI stuff to src/gui/
GUI: Only include the wx includes we need

Instead of the big wx.h.
GUI: Update panelpreviewsound's Doxygen comment
GUI: Move the empty preview panel into PanelPreviewEmpty
GUI: Directories have no file and resource type
GUI: Move resource info panel to PanelResourceInfo