Change how we handle clicking on file:// links on Windows.

Previously we attempted to exec the file. This can be dangerous if
someone sends you a link to a malicious remote file. For now we're
going to open Explorer at the file's location. The user can decide
from there what they want to do--hopefully it'll be more obvious
what they're exec'ing and they can make a more informed decision.

This was a pretty easy change. We already had code to launch explorer.exe
that Eion wrote in this commit:
https://hg.pidgin.im/pidgin/main/rev/4377067bda01

But due to a bug it was only getting triggered if the URI was
"file://file://something"

A possibly better approach is for us to show an "are you sure you
want to do this?" prompt. I don't want to do that in 2.x.y, but
we could do it in default.


REGARDING ESCAPING
We weren't correctly escaping the file path that we passed to explorer.exe.
I believe this would have allowed a remote users to craft links that pass
arbitrary parameters to explorer.exe. I think it is not possible to craft
links that would exec other commands, and the arguments to explorer.exe
look fairly innocuous, so I don't think this is a major problem. But of
course we should fix it--we want to dictate how file:// are opened, we
don't want remote users to be able to dictate this.

The old code called g_shell_quote() to attempt to escape the URI, but it
didn't actually use the return value. Additionally g_shell_quote()
doesn't do what we want. It wrapps the string in single quotes and
escapes single quotes with a backslash. We really just want to escape
double quotes with a double quote.

Incidentally, explorer.exe argument parsing is bat shit crazy [1]. Args
are separated by commas or equals (not spaces). Double-quotes can be
used to wrap an argument but somehow double-quotes within an argument are
ignored. If the first field in an argument is not '/' then the entire
thing is interpreted as a path (until the next comma or equals, I guess?)
For something that's been around for 20 years and is a core piece of
the OS you'd think it would have half-way respectable argument parsing.
Then again, it's Windows.

[1] http://www.geoffchappell.com/studies/windows/shell/explorer/cmdline.htm