Integrate better with Windows' file type associations

[nielsm]

We've had an the-least-that-could-possibly-work approach to file associations previously, but since Windows 95 was new a lot has been added to make user association management more flexible and easier. Let's integrate better with that and try to follow Microsoft's guidelines.

Some links:
 http://msdn.microsoft.com/en-us/library/cc144148%28VS.85%29.aspx
 http://msdn.microsoft.com/en-us/library/cc144154%28VS.85%29.aspx
 http://msdn.microsoft.com/en-us/library/cc144101%28VS.85%29.aspx
 http://msdn.microsoft.com/en-us/library/cc144150%28VS.85%29.aspx

[nielsm]

(In [3760]) Massive file associations patch for Windows. Updates #1037.
Missing is some better migration from old associations. Currently, if the user upgrades from an earlier version, he will be told that "Aegisub is not the current default editor for subtitles" on first start after upgrade, which is wrong at least from the user's point of view. We need to detect old-style associations and clean them up, replacing them with new-style silently.

[nielsm]

Forgot to put in commit message, r3764 to 2.1.8 added code to migrate old associations to new associations. This essentially completes the task for 2.1.8. The installer has also been tested on both XP SP3 and Win7 RC with various combinations of upgrades and fresh installs.

The code should probably be ported to trunk at some point, leaving this tick open for that reason.

[verm]

(In [3768]) Add dialog_associations.cpp to Makefile.am. updates #1037.

[nielsm]

Porting this to the trunk has been moved to #1050 so this ticket can show as fixed for 2.1.8.