Add option to change lesson font

Preferences.cpp
galaxql.cpp
galaxql.h

- New preference 'LessonFont'.
Lesson font is the font shown in the HTML windows and query window.
It is a text value of two font attribute tokens separated by a colon.
<Normal Font Face Name>:<Normal Font Point Size>
eg 'Arial:12'

(RestorePreferences)
- Restore 'LessonFont' preference.

(Galaxql::CreateControls)
- Create the lesson font picker's data object.
- Add an entry in 'Appearance' to change the lesson font.

(Galaxql::OnChangeLessonFontClick) New.
- Summary: Allow the user to change the lesson font.
- Warn that font styles aren't supported.
- Show the lesson font picker, save the user selected font in the
picker's data object and then change the lesson font to the base font of
the user selected font (ie the user selected font sans style info).
- Save the base font name and size in preference 'LessonFont'.

(Galaxql::SetLessonFonts) New.
- Summary: Change the HTML windows and query window to the passed in
normal font face name, fixed font face name and font size.
- Change the passed in font size if it's too small.
- Map the passed in font size to HTML relative sizes.
- Set the HTML windows to the passed in font names and relative sizes.
- Set the query window to the passed in normal font face name and size.