Original Orion as PNG files

This implements the gradients on the entire window border as PNG files. Each border XPM is regenerated, keeping transparent pixels but turning all others to #EEEEEE/inactive_color_2. Then PNG files were automatically generated such that the resulting composite operation between XPM and PNG should yield the original XPM colours. The icon XPMs were cropped to remove extra background pixels, and the window title was moved up by 1 pixel to fix alignment.