Integrate MIT licensed 3rd party Qt translations for missing locales.

* Added locales not shipped with Qt (qt_it.ts, qt_nl.ts, qt_tr.ts)
  taken from the VirtualBox (https://www.virtualbox.org/ticket/2018)
* Extended mklic.pl to be able to add guards to only include certain
  3rd party licenses if a define is set during build.
* Made mach_override specific to OSX
* Integrated new translations into the bundled qt translations build.
* A new define USING_BUNDLED_QT_TRANSLATIONS is now set when building
  with bundled translations. It is used to guard the 3rd party license
  entry for the new translations.
* See mumble/qttranslations/LICENSE for more information on the
  licensing of the new translations.
Add back accelerator keys for Cancel in QDialogButtonBox.

* VirtualBox stripped those keys from dialogs that are universally dismissed by ESC. Without those we do
  not get matching translations for those buttons though.