feat(w.w.slider): Add signals for the dragging state

When trying to hook up the slider to an external value, one would
usually have two data streams:

- `property::value` -> set external value
- external value changed -> `.value = new_value`

The problem is that without manual intervention, these two streams form
a loop, as setting `.value` also emits `property::value`.

The new set of signals is disconnected from the `value` property and its
signal and allows for more fine grained inspection of the dragging
state.

Signed-off-by: Lucas Schwiderski <lucas@lschwiderski.de>

lib/wibox/widget/slider.lua

@@ -244,6 +244,28 @@ local slider = {mt={}}
 -- @beautiful beautiful.slider_bar_active_color
 -- @param color
 
+--- Emitted when the user starts dragging the handle.
+--
+-- @signal drag_start
+-- @tparam number value The current value
+
+--- Emitted for every mouse move while the handle is being dragged.
+--
+-- This signal is only emitted by the user dragging the handle. It is therefore
+-- preferrable over `property::value`, as it cannot create a loop when trying to
+-- hook up the slider as representation of an external value (e.g. system
+-- volume).
+--
+-- @signal drag
+-- @tparam number value The current value
+
+--- Emitted when the user stops dragging the handle.
+--
+-- This signal is emitted when the user releases the mouse button after dragging
+-- the handle.
+--
+-- @signal drag_end
+-- @tparam number value The current value
 
 local properties = {
     -- Handle
@@ -303,6 +325,14 @@ function slider:set_value(value)
     end
 end
 
+--- Returns `true` while the user is dragging the handle.
+--
+-- @method is_dragging
+-- @treturn boolean
+function slider:is_dragging()
+    return self._private.is_dragging
+end
+
 local function get_extremums(self)
     local min = self._private.minimum or properties.minimum
     local max = self._private.maximum or properties.maximum
@@ -504,17 +534,23 @@ local function mouse_press(self, x, y, button_id, _, geo)
 
     move_handle(self, width, x, y)
 
+    self._private.is_dragging = true
+    self:emit_signal("drag_start", self.value)
+
     -- Calculate a matrix transforming from screen coordinates into widget coordinates
     local wgeo = geo.drawable.drawable:geometry()
     local matrix = matrix_from_device:translate(-wgeo.x, -wgeo.y)
 
     capi.mousegrabber.run(function(mouse)
         if not mouse.buttons[1] then
+            self._private.is_dragging = false
+            self:emit_signal("drag_end", self.value)
             return false
         end
 
         -- Calculate the point relative to the widget
         move_handle(self, width, matrix:transform_point(mouse.x, mouse.y))
+        self:emit_signal("drag", self.value)
 
         return true
     end,"fleur")