Use proper font for 'View > Use Default Font'

Use proper font for 'View > Use Default Font'

Symptom:
	Use of the 'View > Use Default Font' menu selection always
	sets the Directory Tree window to the 12-point Courier New font,
	which is not the default font for that window.

Discussion:
	WinMerge has an Options::Font structure that maintains an
	in-memory copy of 'actual' and 'default' values for the font of
	each of the Directory Tree and File View windows.  This structure
	is populated at program initialization by procedure SetDefaults()
	in file Src/OptionFont.cpp.  The 'default' values are generated by
	this procedure, the 'actual' values are loaded from the Registry.

	The existing code only queries the system for the default fixed-
	size font (via the default MIME codepage) and then uses this font
	as the 'default' value for the Options::Font structure for both
	Directory and File windows.  This value is typically Courier New.

	However, WinMerge actually uses the Menu font as the actual Directory
	window font if no other font is marked as 'Specified' in the
	Registry.  The default Menu font (for English, since Windows 7) is
	Segoe UI.  But this use of Segoe UI is never reflected in the
	Options::Font structure, nor in the Registry.

Implementation:
	The procedure SetDefaults() is modified to additionally
	capture the existing Menu font information for the Directory Tree
	window, while continuing to use the codepage MIME font for the
	File View window.

	A "helper" function is added: InitializeLogFont().  It should only
	be used within Src/OptionFont.cpp.  It is documented by comment in
	the Src/OptionFont.h file.

	The information in the Registry for the Options::Font structure
	now always reflects the values of the in-memory structure, and
	the values being used by the windows themselves.

Incidental changes:
* Numerous added or modified comments in Src/OptionFont.cpp
* Reordering of all Registry related code to be in the canonical
	order implied by the layout of the LOGFONT structure itself.
* Explicitly cast the three boolean values (Italic, Underline,
	Strikeout) to boolean to invoke the properly typed Options::Font
	procedures (InitOption, SaveOption)
* Remove two unnecessary String() function references relating to
	font in files Src/DirView.cpp and Src/MergeEditView.cpp