ox-latex: Extend :float attribute to source blocks and normalize its values
* lisp/ox-latex.el (org-latex-src-block): Handle :float attribute. Its
value can be set to "t", "multicolumn" or "nil". Also
remove :long-listing attribute, which is now replaced with :float
nil.
(org-latex--org-table): Replace :float table with :float t.
(org-latex--inline-image): Replace :float figure with :float t.
(org-latex-long-listings): Remove variable.
* doc/org.texi (@LaTeX{} specific attributes): Document new :float values.