Eeschema: fix build error when using wxWidgets 2.8 and other minor fixes.

* Fix string concatenation for _( "Unit" ) + wxT( " " ). This failed
  compiling against wxWidgets 2.8 (thanks Cirilo Bernardo ).
* Base the indentation on a measured observation with explanation
  (98%-ile of name-length found in library). The previously chosen 24
  character indentation was too wide for most.
* Don't use a #define spilling into the global namespace, but use
  a locally defined constant for COLUMN_DESCR_POS.