Change event posting to use a QVector.

Change event posting to use a QVector.

This provides a ~10% improvement to the newly introduced QCoreApplication
event_posting_benchmark (a simple synthetic benchmark of creating a bunch of
events, posting them, and sending the queue).

before:
********* Start testing of QCoreApplicationBenchmark *********
Config: Using QTest library 5.0.0, Qt 5.0.0
PASS   : QCoreApplicationBenchmark::initTestCase()
RESULT : QCoreApplicationBenchmark::signal_slot_benchmark():"1000":
     0.82 msecs per iteration (total: 53, iterations: 64)
RESULT : QCoreApplicationBenchmark::signal_slot_benchmark():"10000":
     8.6 msecs per iteration (total: 69, iterations: 8)
RESULT : QCoreApplicationBenchmark::signal_slot_benchmark():"100000":
     84 msecs per iteration (total: 84, iterations: 1)
RESULT : QCoreApplicationBenchmark::signal_slot_benchmark():"1000000":
     874 msecs per iteration (total: 874, iterations: 1)
PASS   : QCoreApplicationBenchmark::signal_slot_benchmark()
PASS   : QCoreApplicationBenchmark::cleanupTestCase()
Totals: 3 passed, 0 failed, 0 skipped
********* Finished testing of QCoreApplicationBenchmark *********

after:
********* Start testing of QCoreApplicationBenchmark *********
Config: Using QTest library 5.0.0, Qt 5.0.0
PASS   : QCoreApplicationBenchmark::initTestCase()
RESULT : QCoreApplicationBenchmark::event_posting_benchmark():"1000 events":
     0.781 msecs per iteration (total: 100, iterations: 128)
RESULT : QCoreApplicationBenchmark::event_posting_benchmark():"10000 events":
     7.8 msecs per iteration (total: 63, iterations: 8)
RESULT : QCoreApplicationBenchmark::event_posting_benchmark():"100000 events":
     75 msecs per iteration (total: 75, iterations: 1)
RESULT : QCoreApplicationBenchmark::event_posting_benchmark():"1000000 events":
     774 msecs per iteration (total: 774, iterations: 1)
PASS   : QCoreApplicationBenchmark::event_posting_benchmark()
PASS   : QCoreApplicationBenchmark::cleanupTestCase()
Totals: 3 passed, 0 failed, 0 skipped
********* Finished testing of QCoreApplicationBenchmark *********

Change-Id: Ibf56d9526b0a8cbaf171008da4104bb457628172
Reviewed-by: Sergio Ahumada <sergio.ahumada@nokia.com>