fixes for XHTML export. Refs: #113.
The closing (right-hand) view of a StdLinks.Target was ignored, which lead to an incorrect nesting of elements.

Conversion of StdLinks.Link with command Dialog.OpenExternal added.

Only the 5 basic xhtml entities are mapped in XhtmlEntitySets. The extended
Ascii and Unicode characters are converted to utf-8 anyway and no characters can get lost now.

meta tag added for setting the encoding to utf-8 within the generated html document.
This setting is supported by all browsers.

utf-8 conversion delegated to module Strings.

Changes to make the output conform to the XHTML standard by Ivan Denisov.

Default font size and typeface added by Ivan Denisov.

Newlines converted into <br/> by Ivan Denisov.

Problems with empty paragraphs fixed by Ivan Denisov.

Bug with handling of multiple spaces fixed.

Support for additional character and paragraph attributes: underline, strikeout, lead, width, margins etc.

Support for links embedded within text added.

page title set to window title, if available, otherwise 'New Page'.

attribute values and page title encoded according to HTML standards.

Some code cleanups and simplifications resulting in a more readable XhtmlExporter and a more
tolerant behavior in case of nesting/pairing errors in the input text.
Some trivial ASSERTS removed. Unused variable level removed. Id renamed to Anchor.

Signed-off-by: Josef Templ <josef.templ@gmail.com>