Add support for loading fonts from files.

wxFont::AddPrivateFont() can now be used to load a font from a file for the
applications private use. Update the font sample to show this.

Closes #13568.

docs/changes.txt

@@ -109,6 +109,7 @@ All (GUI):
 
 - Allow wxWebView::RunScript() return values (Jose Lorenzo, GSoC 2017).
 - Allow using fractional pen widths with wxGraphicsContext (Adrien Tétar).
+- Add support for loading fonts from external files (Arthur Norman).
 - Improve wxSVGFileDC to support more of wxDC API (Maarten Bent).
 - Add support for wxAuiManager and wxAuiPaneInfo to XRC (Andrea Zanellato).
 - Add support for wxSL_MIN_MAX_LABELS and wxSL_VALUE_LABEL to XRC (ousnius).

docs/doxygen/mainpages/const_cpp.h

@@ -188,6 +188,7 @@ Currently the following symbols exist:
 @itemdef{wxHAS_IMAGES_IN_RESOURCES, Defined if <a href="http://en.wikipedia.org/wiki/Resource_(Windows)">
     Windows resource files</a> or OS/2 resource files are available on the current platform.}
 @itemdef{wxHAS_POWER_EVENTS, Defined if wxPowerEvent are ever generated on the current platform.}
+@itemdef{wxHAS_PRIVATE_FONTS, Defined if wxFont::AddPrivateFont() is implemented.}
 @itemdef{wxHAS_RADIO_MENU_ITEMS,
         Defined if the current port supports radio menu items (see wxMenu::AppendRadioItem).}
 @itemdef{wxHAS_RAW_BITMAP, Defined if direct access to bitmap data using the classes in @c wx/rawbmp.h is supported.}

docs/doxygen/overviews/font.h

@@ -75,4 +75,19 @@ implemented for Windows and Unix (GTK+ and Motif) ports only, all the methods
 are available for all the ports and should be used to make your program work
 correctly when they are implemented later.
 
+@section overview_font_privateinfo Private font information
+
+Sometimes an application needs fonts that are not globally installed on the
+system. On Macintosh/OSX this can be arranged by placing the desired fonts
+within the Application Bundle in Contents/Resources/Fonts and using
+the ATSApplicationFontsPath key to point there. The full details of the
+procedure there can be found as OSX developer resources. For the GTK+ and
+Windows ports it is possible to add TrueType fonts at run-time using
+a sequence of calls to wxFont::AddPrivateFont() to give the names of files
+containing font data, followed by a call to wxFont::ActivatePrivateFonts()
+to complete the process of making the fonts available. These functions
+return false if they fail. They should be called just once before any
+graphics contexts have been created or other activity liable to use fonts
+has happened.
+
 */