Phase out deprecated usage of NOM_DUMP_VAR macro
Revamp logging interface (macros) to support variable # of args

* clock.cpp: Update the time implementation: a) do not output a newline
character at the end of the time stamp; b) use ISO 8601 date & time
standard; c) use TIME_STRING_SIZE in both Windows & Unix
implementations.

* config.hpp.in: Use new logger interface, NOM_LOG_INFO, NOM_LOG_ERR,
NOM_LOG_TRACE, NOM_STUBBED -- these accept a variable number of
arguments arguments.

* config.hpp.in: Introduce NOM_DUMP_VAR macro; accepts a variable
number of arguments. The existing NOM_DUMP macro is unmodified.

* config.hpp.in: NOM_STUBBED logging is now reported under
<identifier>_LOG_ERR level, with the rationale that it will be more
likely to be caught by my poor eyes.

* config.hpp.in: NOM_DUMP_EXT macro has been removed; obsoleted.

* config.hpp.in: Clean up the order of header files.

* Several misc header files had to be updated -- adding SDL.h -- after
it was removed from system/clock.hpp.
Introduce macros example app
UIBoxLayout: Fix UISpacerItem & internal layout spacing bugs

* Button: Use the label's width as the width of the returned Size in
the size_hint method.
* Button: Return the minimum size of the widget when all else fails
inside the size_hint method.

* ListBox: Use nom::DEFAULT_FONT_SIZE within both size_hint and update
methods.
* ListBox: Use the font point size of the UIStyle when available; this
is done in both ::size_hint && ::update.

* examples/gui_listbox: Introduce several assertions for ensuring
layout logic is sane. (These will be relocated soon!)

* examples/gui_button: Introduce several assertions for ensuring
layout logic is sane. (These will be relocated soon!)
Update examples template file
Introduce Cursor class (system cursors)

* RenderWindow: Introduce mouse_focus method.

* Rename gui/Cursor to gui/GameCursor & mark it deprecated.

* Introduce examples/cursors.cpp.
Introduce ColorDatabase && SystemColors interface
ListBox: Implement wheel & click focus policies

* Cursor: Declare mouse_state method static.

* Button: Set focus policy to FocusPolicy::StrongFocus.

* UIStyle: Introduce selected_font_color getter & setter.
* ListBox: Use UIStyle's selected_font_color when available.