Refactored the library build procedure into a more modular format
using new cmake features.

Adds Hunter to the CMake setup. Can be enabled buy calling cmake with -DUSE_HUNTER=ON.

Also commented out freeimage project since it is not  yet
available via hunter.

This resolves naming conflicts when static library
of forge is created.

Also marked some varaibles as advanced as they don't need to
be modified for most of the use cases.

hunter packages are polluting the cmake cache space and thus have
been mark as advanced as one wouldn't need to modify then in most
cases.

Earlier, there were two separate flags. From, now on
just FG_USE_STATIC_CPPFLAGS would suffice

The new lines are creating discontinued reading experience in GUI installers.

Variables that fit the pattern `FG_INSTALL_*` are marked
as advanced

* Install version header as part of the headers component
* Remove configure_file command for NSIS template file on non-windows platforms.
* Move glsl_shader headers under `PROJECT_BINARY_DIR/include`
* Remove unused target properties from examples targets
* Merge ConditionalDirectory and IfNotThisThenInclude scripts into
  InternalUtils cmake script.

This enables CMake to find Forge automatically if it is installed
in standard location on Windows platform.

The version prior to this change, 3.3.0-p4, increased minimum cmake
version to 3.8. To bring it down to 3.0, the version of glfw from hunter
had to be changed to 3.3.0-p2

OpenCL package has been removed from hunter, not sure why though.
It used to be present.

* Change appveyor config to use vcpkg
* Update travis config for 18.04 update
* change build version format in appveyor config