Textview, a simple text viewer for big files (e.g., Qemu logs)

[laerreal]

Minimum features set for now.

v2:

• fixup coding style and typos
• add docstring for OffsetsStream.__init__
• use divmod in offset2line_offset
• extend SoftStream docstring
• fixup scrolling by mouse wheel under Windows
• OS independent keyboard bindings based on KeyboardSettings
• fixup OffsetsStream doctest about 0xDEADBEEF under Windows Py2
• TextCanvas: utf-8 encoding with "replace" error handling (fixes troubles with \ in raw data).

v3:

• fixup coding stile and typos
• wrap_wheel_handler tested on Windows 10
• HKCombobox use OS-independent COPY_KEYCODE too

[laerreal]

v3 is ready