Updated OctaveX plugin

Dear all,
in the meantime that the official Octave plugin is developed, you may find useful an updated OctaveX plugin (zipped archive below).

Main changes:
-  now the plugin has to be installed in the TEXMACS_HOME_PATH folder (e.g.  ~/.Texmacs/plugins/octaveX in linux and OSX, C:\Users\USERNAME\AppData\Roaming\TeXmacs\plugins\octaveX in Windows), see figure below. Note: if necessary update the plugin list with the menu command Tools/Update/Plugins,
-  corrected a bug on Windows (when TeXmacs was launched by double-clicking on documents that are on a different disk than the main program, OctaveX initialization failed),
-  corrected a bug on OSX (when the Octave.app bundle was used to launch octave, the qt graphics toolkit was not correctly initialized),
-  added a non-verbose mode, with most of the messages suppressed,
-  added a menu with some OctaveX related commands, in the ‘user provided icons’ bar,
-  moved from environment variables to the octave prefs framework to store preferences,
-  help updated.

Best,
Lorenzo