Add a class to compare .dll files for *real* changes

... i.e. *not* the time stamp nor the .pdb GUID.

The problem is that recompiling *identical* code on Windows does not
result in byte-identical .dll files. The reason is that the .dll file
format *defines* time stamps to be part of the format. Even without
timestamps, there is another problem: disregarding the debug option,
Microsoft Visual C compiler still incorporates .pdb information, always,
including a Global Unified IDentifier that changes *every single time*
source code is compiled.

The DllFile class is designed to analyze .dll files and determine
whether they contain *functional* changes as opposed to circumstantial
ones, such as time stamps or GUIDs.

We need this functionality to be able to avoid uploading "different"
files every night when uploading Micro-Manager's nightly build.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>