...that (indirectly) allocates memory via rtl/alloc.h, thereby causing the
rtl_cache_wsupdate_init thread to be spawned before main, as on Mac OS X that
would interfere with the code in sal_detail_initialize to close all file
descriptors >= 3 -- on Mac OS X the pthreads implementation makes use of KQUEUE
file descriptors.

* This commit removes enough global static data to make ui-preview work again on
Mac OS X (where it crashed at startup when the main thread closed the KQUEUE fd
used by pthreads implementation threads).  gengal uses further static data (at
least from module sb), so needs further clean-up.

* Avoiding global static instances derived from class Application required the
introduction of vcl/vclmain.hxx.

* That the vcl library was linked against the static vclmain library (which only
provides an implementation of main) appears to me to be a historic relic (all
executables should either include a SAL_IMPLEMENT_MAIN or link against vclmain),
so I removed that.

Change-Id: I048aa616208cb3a1b9bd8dcc3b729ba1665729bd