This updates the gla11y tool

- mnemonic-widget is actually an alias for mnemonic_widget
- ditto for tooltip-text vs tooltip_text
- ditto for placeholder-text vs placeholder_text
- glade introduced a notation for specifying the role of a widget, which can
  be used to make a label static for instance (thus not orphan).
- --help is fixed to use the standard formatting.
- Remove the documentation of unexisting -i option
- Add the documentation of -P option
- Add --help/-h and --version options for completeness.

Change-Id: I8510ba9c5a877c05edd3af305b4e7fe1f105acd5
Reviewed-on: https://gerrit.libreoffice.org/52973Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Thorsten Behrens <Thorsten.Behrens@CIB.de>