- replace FL_OVERRIDE and NULL with 'override' and 'nullptr', resp.
- remove useless global variable (declare it locally in main)
- standardize callbacks using standard callback signatures
- rename some callbacks with more appropriate names
- fix layout of image display and image size info
- improve alignment of text display
- set window size_range (minimal window size)
FL_OVERRIDE is defined as `override` for C++11 and higher
FL_OVERRIDE is defined as `override` for VisualC 2015 and newer
Don't interfere with Fl_Widget::override()
