doc: Add a default widget documentation.

This will avoid copy/pasting.

This assume the widget use the property system.

docs/common/widget.ldoc

@@ -0,0 +1,41 @@
+
+--- Get a widex index.
+-- @param widget The widget to look for
+-- @param[opt] recursive Also check sub-widgets
+-- @param[opt] ... Aditional widgets to add at the end of the "path"
+-- @return The index
+-- @return The parent layout
+-- @return The path between "self" and "widget"
+-- @function index
+
+--- Get all direct and indirect children widgets.
+-- This will scan all containers recursively to find widgets
+-- Warning: This method it prone to stack overflow id the widget, or any of its
+-- children, contain (directly or indirectly) itself.
+-- @treturn table The children
+-- @function get_all_children
+
+--- Set a declarative widget hierarchy description.
+-- See [The declarative layout system](../documentation/03-declarative-layout.md.html)
+-- @param args An array containing the widgets disposition
+-- @function setup
+
+--- Force a widget height.
+-- @property forced_height
+-- @tparam number|nil height The height (`nil` for automatic)
+
+--- Force a widget width.
+-- @property forced_width
+-- @tparam number|nil width The width (`nil` for automatic)
+
+--- The widget opacity (transparency).
+-- @property opacity
+-- @tparam[opt=1] number opacity The opacity (between 0 and 1)
+
+--- The widget visibility.
+-- @property visible
+-- @param boolean
+
+--- Set/get a widget's buttons.
+-- @param _buttons The table of buttons that should bind to the widget.
+-- @function buttons