input/Stream: remove attribute "cond", replace with handler interface

This adds a bit of overhead, but also adds flexibility to the API,
because arbitrary triggers may be invoked from that virtual method
implementation, not just Cond::signal().

The motivation for this is to make the handlers more dynamic, for the
upcoming buffering class utilizing ProxyInputStream.

Makefile.am

@@ -1382,6 +1382,7 @@ libinput_a_SOURCES = \
 	src/input/InputStream.cxx src/input/InputStream.hxx \
 	src/input/Ptr.hxx \
 	src/input/InputPlugin.hxx \
+	src/input/Handler.hxx \
 	src/input/RemoteTagScanner.hxx \
 	src/input/ScanTags.cxx src/input/ScanTags.hxx \
 	src/input/Reader.cxx src/input/Reader.hxx \