qt: don't install translation files by default

Commit 93917b69 (2013.11) introduced
the installation of the binary .qm translation files, unconditionally.

However, the installed size is very large for some embedded systems
(about 8 MB), and they are not needed in many cases.

In order to avoid such a waste of storage space, commit
2ff32941 (2014.08) made this
installation an option that could be disabled. For backward
compatibility, the option default was set to yes.

Given the size of these files, and that they had never been installed
by Buildroot versions before 2013.11, change the default to not
installing them.

Interested users can still activate it as needed.

Also update and improve the help text.

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Danomi Manchego <danomimanchego123@gmail.com>
Cc: Danomi Manchego <danomimanchego123@gmail.com>
Cc: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>