Revise svg icons for the math toolbar

Revise svg icons for the math toolbar

- Use paths, not fonts. Have a look at the sqrt icon in the image attached
  here http://thread.gmane.org/gmane.editors.lyx.devel/154179/focus=154183
  to see what may happen when a font is not present on a given system.
- Make uniform the height of the parentheses.
- Move a couple of icons to the oxygen subdir (I was not sure it was Ok
  replacing them. If yes, they can be simply deleted).