Added facility to export custom hints for modules

Added ${vtk-module}_WRAP_HINTS as a module specific hints file, to allow
modules to set a custom wrapping hints file (as external projects such
as ParaView have done for many years).

Also cleaned up the installation code to ensure the hierarchy and hints
files are installed when appropriate.

Change-Id: Ie019fab326a3072a33bdc2aaf066180b2fa32eb1
Fixed copy/paste error - WRAP_HINTS install path

Thanks to Utkarsh for spotting the error, we need a path for the build
tree and a path for the install tree for any custom hints file.

Change-Id: I57b49a7a91da75286136652fdb65d89b54b15217
These should be appending the WRAP_HINTS to the vars

The previous version would have caused bugs when there was a custom
hints file and some CMake code to export.

Change-Id: Ie33b73be4505c0f483f4f2d9727cce744fee1005
Merge topic 'python-custom-hints' into master

ed0d4e81 These should be appending the WRAP_HINTS to the vars
e5323fc7 Fixed copy/paste error - WRAP_HINTS install path
95d4ca64 Added facility to export custom hints for modules