The values in mach_timebase_info_data_t should be specified in terms of
'ticks'. On some machines, ticks may correspond to nanoseconds, but this
is not guaranteed.

Previously, if `stopTimer()` was called from within
`hiResTimerCallback()`, a call to `isTimerRunning()` immediately
following the call to `stopTimer()` would return true instead of false.

This patch fixes the issue, and adds some tests to verify the new
behaviour.

In very optimised builds, SHCore.dll may not be automatically loaded
into the address space of the calling executable. In these cases, the
call to `GetModuleHandle` will fail. Adding the call to LoadLibrary
ensures that the dll is loaded into the program's address space, which
will allow the call to GetModuleHandle to succeed.

Previously, if `loadImpulseResponse` was called before `prepareToPlay`,
the IR wasn't guaranteed to have loaded before the first call to
`processSamples`.

Now, we flush the queue of pending IR-load commands during
`prepareToPlay`, which should ensure that the most recently-loaded IR is
ready to use immediately.

This change affects the PostVista version of the Windows FileChooser.

The default main JuceActivity for JUCE apps calls appNewIntent() in its onNewIntent() implementation which is defined in juce_gui_extra. This commit moves the implementation into juce_gui_basics.