Add <span> tag and limited support for CSS styles to wxHTML.

Parse "style" attributes of the HTML tags and honour those of them that can be
mapped to the HTML 3 attributes.

Also add a handler for <span> tag.

Closes #10631.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@64443 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

docs/doxygen/overviews/html.h

@@ -543,6 +543,7 @@ P               ALIGN=[alignment]
 PRE
 SAMP
 SMALL
+SPAN
 STRIKE
 STRONG
 SUB
@@ -576,5 +577,12 @@ U
 UL
 @endcode
 
+@subsection overview_html_suppstyles_list List of supported styles
+
+wxHTML doesn't really have CSS support but it does support a few simple styles:
+you can use @c "text-align", @c "width", @c "vertical-align" and @c
+"background" with all elements and for @c SPAN elements the @c "color", @c
+"font-size" and @c "font-weight" are additionally recognized.
+
 */