Changes needed to accommodate building libevoral with MSVC

Note that class 'MusicalTime' needn't be exportable (with a little work, this could be reverted if it affects the other builds)
Remove a commented out line in 'pbd/stacktrace.h'

(was probably just needed for testing at some stage)
Simulate 'trunc()' which isn't available in MSVC

(needed by 'gtk2_ardour/editor.cc')
Now that we're building pango's language modules statically (i.e. no longer as DLLs) it's not necessary to edit the user's 'pango.modules' file
Add a newly introduced header file to our MSVC project (gtk2_ardour)