doc: Add gears.object common documentation

docs/common/object.ldoc

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+
+--- Disonnect to a signal.
+-- @tparam string name The name of the signal
+-- @tparam function func The callback that should be disconnected
+-- @see add_signal
+-- @function disconnect_signal
+
+--- Emit a signal.
+--
+-- @tparam string name The name of the signal
+-- @param ... Extra arguments for the callback functions. Each connected
+--   function receives the object as first argument and then any extra arguments
+--   that are given to emit_signal()
+-- @function emit_signal
+
+--- Add a signal to an object. All signals must be added before they can be used.
+--
+-- @tparam string name The name of the new signal.
+-- @function add_signal
+
+--- Connect to a signal.
+-- @tparam string name The name of the signal
+-- @tparam function func The callback to call when the signal is emitted
+-- @see add_signal 
+-- @function connect_signal
+
+--- Connect to a signal weakly. This allows the callback function to be garbage
+-- collected and automatically disconnects the signal when that happens.
+--
+-- **Warning:**
+-- Only use this function if you really, really, really know what you
+-- are doing.
+-- @tparam string name The name of the signal
+-- @tparam function func The callback to call when the signal is emitted
+-- @function weak_connect_signal