Consistently hide the parts that require xcb-xkb

It was inconsistent. This also solves a warning from Clang 3.4:
error: private field 'vmod_masks' is not used [-Werror,-Wunused-private-field]

Change-Id: I6be9f7ef56dffe6df2be3beb984c2d82d3808403
Reviewed-by: Gatis Paeglis <gatis.paeglis@digia.com>
Use Q_STATIC_ASSERT to check that the array has the right size

No need to sprinkle the code with Q_ASSERT. We don't want this to fail
to the user anyway. By using a Q_STATIC_ASSERT, a failure will be
immediately reported to the developer.

This also solves a warning found by Clang 3.4:
error: unused variable 'pageSizesCount' [-Werror,-Wunused-const-variable]

Change-Id: I79cf72c64242ad395276ce4360c59ad81112d9eb
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Fix capacity reservation for shared QByteArray

We can squeeze, but not by discarding elements. Make sure the size of
the object stays intact after changing the reserved capacity.

I've also added unit tests for other containers, just to be sure.

Task-number: QTBUG-37750
Change-Id: I5135b095943b7589423c51cebcb52af792468e61
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>