Possibility to adjust the hidden content behaviour in the send e-mail
functionality via option under the e-mail program setting

Signed-off-by: Stephan Bergmann <sbergman@redhat.com>, taking just the parts
that store the configuration property within /org.openoffice.Office/Security
(rather than /org.openoffice.Office/Common/ExternalMailer) and throwing away
the parts overlapping with previously picked
<https://gerrit.libreoffice.org/#/c/14994/> "fdo#87441-Send document as Email
removes invisible content."  Plus making the configuration property
non-nillable, -Werror,-Winconsistent-missing-override, loplugin:salbool, and
removal of trailing whitespace.

Conflicts:
	cui/source/options/optinet2.cxx
	cui/source/options/optinet2.hxx
	cui/uiconfig/ui/optemailpage.ui
	sw/source/uibase/app/docsh2.cxx

Change-Id: I1fb70418c5fe8011a86a379c2e75eae9ad3e18c3