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.
Fix typo in mklic.pl
GlobalShortcutWin: try to make WinHooks do less work.

This change switches GlobalShortcutWin over to a slightly different model
in an attempt to combat the WinHook timeouts users have been reporting.

Previously, the DirectInput poller and the WinHooks event handling lived in
the same high-priority thread, and both of them processed all incoming button
events. Since both of them processed all events and emitted them using
GlobalShortcutEngine's handleButton method, they had to live in the same
thread in order for us to control the order of the signal dispatch.

Having the WinHooks live in the same thread as the DirectInput polling meant
that the hooks could potentially be blocked every 20 ms by the DirectInput
poller's timer event, which wasn't really desirable.

This new model splits up the DirectInput poller and the WinHooks event handling
into two separate, high-priority threads, each with different responsibilities.

The DirectInput poller is now the main source of events. This is where the actual
global shortcut presses are handled. Having DirectInput responsible for all
global shortcut buttonUp and buttonDown events allows us to use the WinHooks
thread *just* for filtering suppressed keys.

This design allows our WinHooks to have their own thread and event loop, meaning
they won't have to fight the DirectInput polling timer. Also, since the WinHooks
now have a more restricted set of tasks, they're able to do less work, hopefully
making it harder to trigger a timeout.
GlobalShortcutWin: revert 242af40b to restore keyboard suppression.

When our WH_KEYBOARD_LL hook suppressed keypresses, DirectInput couldn't
read them.

Let's revisit this optimization for 1.2.5. We can always refer users
having issues with hook timeouts to the registry hack.
setLiveConf for opusthreshold and channelnestinglimit didn't reset on
empty string (#938)
Introduce concept of NULL plugin to disable plugins.

* NULL plugins return NULL in the getMumblePlugin() call silently excluding them from the plugin list.
* NULL css, dods, tf2 and hl2dm plugins as they have been replaced by link support in the source engine.