add spinbox, not working at all
spinbox compiling passed

Added a sample in multi_windows program.
spinbox runnable

Spinbox in multi_window program works. Default height is changed to make
it looks like a spinbox. Spinbox widget starts to track changes in its
text_line sub-widget.

Next thing to do is to implement the event subscribing interface like
that of text_line widget.
add event subscribing interface

Currently only one event is available: SPINBOX_CURRENT_CHANGE. More
events can be added easily as needed.
Merge branch 'spinbox' into better-widgets
adapt spinbox to new coding style

Use common code in widget.c.

Also removing font data from spinbox, as text_line sub-widget already
has it.