libcamera: base: signal: Support connecting signals to functors

It can be useful to connect a signal to a functor, and in particular a
lambda function, while still operating in the context of a receiver
object (to support both object-based disconnection and queued
connections to Object instances).

Add a BoundMethodFunctor class to bind a functor, and a corresponding
Signal::connect() function. There is no corresponding disconnect()
function, as a lambda passed to connect() can't be later passed to
disconnect(). Disconnection typically uses disconnect(T *object), which
will cover the vast majority of use cases.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Umang Jain <umang.jain@ideasonboard.com>

Documentation/Doxyfile.in

@@ -878,6 +878,7 @@ EXCLUDE_PATTERNS       = @TOP_BUILDDIR@/include/libcamera/ipa/*_serializer.h \
 
 EXCLUDE_SYMBOLS        = libcamera::BoundMethodArgs \
                          libcamera::BoundMethodBase \
+                         libcamera::BoundMethodFunctor \
                          libcamera::BoundMethodMember \
                          libcamera::BoundMethodPack \
                          libcamera::BoundMethodPackBase \