xlfont load a font in the graphic context or query loaded font

## Syntax

    xlfont(font-name)
                xlfont(font-filename)
                xlfont('reset')
                xlfont(font-name,font-id)
                xlfont(font-filename,font-id)
                xlfont(font-name,font-id,bold)
                xlfont(font-name,font-id,bold,italic)
                fonts=xlfont('AVAILABLE_FONTS')
                fonts=xlfont()

## Arguments

font-name

  string, name of the font family.

font-filename

  string, filename of a true type font.

font-id

  integer >= 0 .

fonts

  a column vector of font names.

bold

  a boolean %t if bold

italic

  a boolean %t if italic

## Description

Without any argument, `xlfont()` returns the list of currently loaded fonts.

xlfont('AVAILABLE_FONTS') returns list of fonts available on your system.

xlfont('reset') reset to initial index list of fonts.

With arguments, `xlfont` is used to load a new font at different sizes in the graphics context.

Default family fonts are "Monospaced" (0), "Symbol" (1), "Serif" (2), "Serif Italic" (3), "Serif Bold" (4), "Serif Bold Italic" (5), "SansSerif" (6), "SansSerif Italic" (7), "SansSerif Bold" (8), "SansSerif Bold Italic" (9). These default fonts are automatically loaded when needed and so `xlfont` is not really required for them. In fact `xlfont` is essentially useful to load a new font.

## Examples

    xlfont('reset');
    xlfont()

    // Caution : this example may not work if your system have not Monospaced font.
    xlfont("Monospaced",10,%t,%t);
    xstring(1,0,'A title');
    figure_entity = gcf();
    axes_entity = figure_entity.children;
    title_entity = axes_entity.children;
    title_entity.font_style = 10;

    xlfont('reset');