Introduced OFoptional, implementing optional types.

OFoptional is implemented based on the C++14 proposal for std::optional
(N3690) and is similar to Boost.Optional.
Fixed issue with text alignment in PatternLayout.

Fixed issue with text alignment in oflog's PatternLayout. After switching to
log4cplus version 1.1.0 the format specifiers for left and right justified
text of a particular width did not work any more. This has been fixed by
returning to the implementation of the previous log4cplus version 1.0.x.
Also removed trailing spaces and fixed some typos.
Fixed some Doxygen related issues (and a typo).
Renamed DoxygenLayout.xml to comply with filename conventions.
Minor fixes and workarounds for Microsoft's compilers.

Microsoft Visual Studio again shows its lack of standard compliance,
especially in the hard bug where calling "operator->()" does
something different than calling "->".
Consistently use "presInfo" for variables/parameters.

Consistently use "presInfo" as the name of parameter and local variables of
type DcmPresentationContextInfo, both in header and source files.
Added support for Supplement 159 to data dictionary.

Added support for the final text version of Supplement 159
(Radiopharmaceutical Radiation Dose Reporting) to the data dictionary.
Added support for new UID from Supplement 159.

Added support for new SOP Class UID from Supplement 159 (Radiopharmaceutical
Radiation Dose Reporting).
Added support for new SR IOD from Supplement 159.

Added full support for the new Radiopharmaceutical Radiation Dose SR IOD
introduced with Supplement 159.
Added DcmSCP::abortAssociation() method.

Added method that allows one to abort an association by sending an A-ABORT
request from the SCP, i.e. classes derived from DcmSCP can now abort the
association in case of severe errors.
Fixed typo and updated copyright date.
Removed meaningless layout file setting.

Removed setting a value for "LAYOUT_FILE" because it is meaningless for man
pages and produces only warnings in the "manpages.log" file.
Renamed Doxygen's layout file again.
Made sure that header file is parsed by Doxygen.
Group condition constants in Doxygen output.
Introduced OFStandard::dropPrivileges().

This method centralizes the code for dropping root privileges when
running in setuid root mode.
Tools now use OFStandard::dropPrivileges().

Modified applications and support classes to call
OFStandard::dropPrivileges() instead of setuid(getuid()).
Avoid full directory path in Doxygen footer.

Avoid full directory path in Doxygen footer when HTML generation is initiated
from CMake.
Replaced CMake variable in Doxygen config file.

Replaced CMake variable "@CMAKE_CURRENT_SOURCE_DIR@" in Doxygen configuration
file by existing substitution variable "@DOXYGEN_INPUT_DIR@".
Removed outdated comment on DICOM 2008 standard.