boost signals2 with clang and C++11 require at least boost 1.55

https://svn.boost.org/trac/boost/ticket/8260

Change-Id: I06b4d45b91c99d1152fa79e419346c0668573bca
avoid warnings with boost signals2

Change-Id: I003f4f951d3b4007bc11ac957ac88bf92d87aab3
convert ListBox's ImplBtn to use boost signal2

Change-Id: I18680e47688da02c9e211bf7f8d861622735e749
convert UserDrawHdl to boost signals2

Note that ImplListBox::userDrawSignal is connected to
ImplListBoxWindow::userDrawSignal to forward the signal, just like
the Link was previously forwarded.

Change-Id: Ic8fc6f51e309bc34fb4e39c13745302649991d0a
convert Edit autocomplete Hdl to boost signals2

Here it also keep track of the connection, as it needs to be explicitly
disconnected in one place.

Change-Id: Id3e2882106ae55c2d880898956a36f84d3ce70ef