Add CMake option to enable/disable short file names in Doxygen

Commit 3f363ae enabled short file name generation in Doxygen to address filesystem limitations that arose while packaging PCL. However, for the purposes of online documentation, short names are not acceptable as they change with every build. This makes referring to function/class documentation impossible.

This pull request adds a CMake option `DOXYGEN_USE_SHORT_NAMES` which affects the corresponding Doxygen option. By default it is set to `OFF`, i.e. generated file names will be long and descriptive, just like this used to be prior to the aforementioned commit.