migrate OS X from NSAutoreleasePool to @autoreleasepool

Offending files found via:
$ find . -type f -name '*.h' -o -name '*.cpp' -o -name '*.mm' -o -name '*.m' -exec grep -l "NSAutoreleasePool" {} \;

While it would appear as though a lot of code has changed, really it is largely due to an extra level of indentation required by @autoreleasepool. Additionally, all lines touched should be normalized to use tabs instead of spaces - I'm not a fan of tabs, but I'd rather be consistent than not.

One API change made in ofSystemAlertDialog() to migrate off a mega deprecated NSAlert API.

One file was updated that targets iOS, seemingly missed in #2888.
Update ofxAndroidVideoPlayer.cpp

If the texture is unloaded by "unloadTexture()" method and if the Android app goes then to pause state; when returning to resume state a Fatal Error is occuring because the unloaded (unavailable) texture cannot be reloaded.

Due to this bug I added:

    texture.texData.bAllocated = false;

line to the end of unloadTexture() method.
Update ofxAndroidVideoPlayer.cpp

I put the unloadTexture() method to its old version and I add a new "removeTexture()" method to the code.

ps. The new method has to be added also to the header file.
Update ofxAndroidVideoPlayer.h

I added the proposed removeTexture() method to the header file as well.
fix compile error due to variable scope

The width and height variables are used outside the Autorelease Pool Block, oups. Indentation within the block scope was also sorted to be consistent with the method.
Merge pull request #3132 from noyanc/patch-1

Update ofxAndroidVideoPlayer.cpp
Merge pull request #3058 from pizthewiz/osx-autoreleasepool

migrate OS X from NSAutoreleasePool to @autoreleasepool
Update ofSystemUtils.cpp

stores the OpenGL context and restores it after a load / save dialog is popped up. closes #2800