doc: Share the smallest common denominator `wibox.layout` doc

docs/common/fixed.ldoc

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+--- Set a widget at a specific index, replace the current one
+-- @tparam number index A widget or a widget index
+-- @param widget2 The widget to take the place of the first one
+-- @treturn boolean If the operation is successful
+-- @name set
+-- @class function
+
+--- Replace the first instance of `widget` in the layout with `widget2`
+-- @param widget The widget to replace
+-- @param widget2 The widget to replace `widget` with
+-- @tparam[opt=false] boolean recursive Digg in all compatible layouts to find the widget.
+-- @treturn boolean If the operation is successful
+-- @name replace_widget
+-- @class function
+
+--- Swap 2 widgets in a layout
+-- @tparam number index1 The first widget index
+-- @tparam number index2 The second widget index
+-- @treturn boolean If the operation is successful
+-- @name swap
+-- @class function
+
+--- Swap 2 widgets in a layout
+-- If widget1 is present multiple time, only the first instance is swapped
+-- @param widget1 The first widget
+-- @param widget2 The second widget
+-- @tparam[opt=false] boolean recursive Digg in all compatible layouts to find the widget.
+-- @treturn boolean If the operation is successful
+-- @name swap_widgets
+-- @class function
+
+--- Get all children of this layout.
+-- @param layout The layout you are modifying.
+-- @return a list of all widgets
+-- @name get_children
+-- @class function
+
+--- Reset a ratio layout. This removes all widgets from the layout.
+-- @param layout The layout you are modifying.
+-- @name reset
+-- @class function