Add in missing ,,,,

This separates the mantidplot and mantidworkbench dependencies into
different branches. It does the same thing for (ubuntu) python2/3
dependencies. Ubuntu 16.04 is now specifically checked for with all the
other newer ubuntus being considered default. Also did significant
changes to ubuntu requirements to form them more closely to how rhel
requirements are.

This make several changes to the mantid python executable:
    - Changes the templated executable to be called mantidpython.in to
    follow the conventions we've followed elsewhere
    - Moves the executable into the bin folder
    - Fixes the start script so that it can be run in a developer build.

The Windows dependencies need downloading before Eigen is configured

Refs #0

Also got rid of the brittle inspection of the architecture
and distribution favoring `CPACK_RPM_PACKAGE_RELEASE_DIST`
introduced in cmake 3.6.

https://cmake.org/cmake/help/v3.6/module/CPackRPM.html

Refs #0

This also removes support for packaging trusty (14.04)

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
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# Lines starting with '#' will be ignored, and an empty message aborts
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Required for mslice and will be required for
the workbench in the future

Refs #21679

QWebEngine replaces QWebKit in Qt>=5.5.
Refs #20381

Refs #20381

The Qt4/Qt5 plugins cannot be loaded into the same process so
need to be separated at build time.
Refs #20288

Along with disabling environment variable updates by default.
Developers will not then accidentally build a release package
when building locally.

Refs #20288

Refs #20288

Refs #20288

Refs #20288