plugins: add magic values for all previously supported ABIs (and a couple of new ones) to mumble_plugin.h.

Add all previous magic values we've used since at least 1.2.0.
(Though I think the MUMBLE_PLUGIN_MAGIC value for MSVS2008 x86
goes back even further!)

This also adds a new magic value for the MSVS2013 x64 ABI, and
a generic default for "unsupported" ABIs.

Unsupported in this context means platforms/ABIs that we aren't
distributing plugins for via the auto-update mechanism. Those
platforms typically only have a single plugin, which is the
"link" plugin distributed with Mumble itself.
plugins: add missing 'elif'; fix indentation of MSVS2008 'if' block.
celt-0.7.0-src, .gitmodules: point the submodule to our own CELT 0.7.0 code base to fix Windows/amd64 build of celt-0.7.0.
Make Mumble and Murmur build on Windows/amd64.
Log IP address on failed login

* Show the IP address of the rejected connection in the mumble-server log.
** Allows use in ban-software which scans logs (requested for fail2ban).
*** Note that the address format will differ with activated IP obfuscation.
* Applied patch from https://sourceforge.net/p/mumble/bugs/861/
which implements the FR #1303 and fixes debian bug 627139 [1]
* This patch was already automatically applied to our Ubuntu PPA builds [2]

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=627139
[2] https://github.com/mumble-voip/mumble-ubuntu-ppa/blob/master/patches/02-reject-with-ip-in-log.diff
Overlay: Robustness and code clarity

* Add an additional map.erase which is not strictly necessary but more
consistent and elaborative (and more robust with changes).
Overlay: Code commenting
DEV: Add questions.
Overlay: Add code comment
Initialize member variables in constructor.
Overlay: Code changes for clarity/robustness (PID handling)

* It is unclear why uiPid would be assigned a non-null value. The if-check
in Overlay.cpp seems useless in that case.
* Check for window handle being NULL, which could lead to
GetWindowThreadProcessId returning it as NULL - although the MSDN doc does
not explicitly say so.
* Check for pid being 0 as well, to preserve behaviour. This is the robust
way.
* Do the same check for MAC, although - lacking an API - I could not
verify if GetProcessPID is specified to return 0 in specific cases.
Code refac: Decrease method visibility (introduce scopes)
Code: Separate logic into methods

* Unfortunately, the readHeader method has a 4-value enum return value
right now in order to keep the old behaviour.
Refac: Debug log texts to common style

* Word win API function failures like in d3d9.cpp "xy failed".
Update LICENSE file
Add question comment
Rename class Mutex to ScopedCriticalSection. Add additional synchronizations
Refactor: Make code more consistent
Refactoring overlay: Naming OverlayMessagePipe

* Rename class Pipe to more specific OverlayMessagePipe.
Refactor: Adjust critical sections

* Completeness
* Minimal
* Similiar commenting