qt: make installation of translation files optional

Commit 93917b69 introduced the
installation of the binary .qm translation files, unconditionally.
This patch introduces an option to disable this behavior, saving
almost 8MB of space.

[Thomas: rename option to BR2_PACKAGE_QT_TRANSLATION_FILES instead of
BR2_PACKAGE_QT_INSTALL_TRANSLATION_FILES, and move its definition
around the installation of examples/demos rather than in the middle of
the options for the different modules.]

Signed-off-by: Danomi Manchego <danomimanchego123@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>