Attempt to make vcl::Timer use boost's Signals2

[Kohei Yoshida]

On Tue, 2014-11-18 at 23:50 +1100, Chris Sherlock wrote:
> Hi all, 
> 
> 
> I'm attempting to get Timer to use boost's Signals2. 

When you do that, could you apply "pimpl" pattern to hide its boost
usages from the header?  Timer is used in a lot of call sites, and
adding yet another boost template to such a public header would further
degrade build time and potentially increase the size of the binary.

I've started working on applying pimpl patterns to those public headers
which have recently been converted to use boost, STL etc (these are
quite many in vcl now).  Not adding new ones would be greatly
appreciated.

Thanks!