FMSLogo Code

Fix Bug #570; CHDIR no longer reports success when it fails and vice-versa.
This regression was introduced in FMSLogo 8.0.0 when 'chdir' was replaced with 'wxSetWorkingDirectory'.
chdir returns 0 on success, but wxSetWorkingDirectory returns 0 (false) on failure.

Remove vestigial #include directives

Fix build break on GNU/Linux when compiling without precompiled headers

Add some comments

Fix typo: "procedures operator on" -> "procedures which operate on"

Standardize on "drawing surface" as a term («área de dibujo» in Spanish)

Fix Bug #566; Parenthesized expressions no longer interfere with parsing subexpressions with infix operators.
For example: "(POWER 1 + BITOR 1 2 4)"

Add some comments to paren_infix.

Add screenshots to the sample instructions for modifying the the desktop shortcut.

Add an explicit regression test for bug #506.