LibGUI: Add ValueSlider widget

ValueSlider is a more generalized version of OpacitySlider when we need
a slider with values displayed. It will always show the current value
with a user defined suffix.

Userland/Libraries/LibGUI/Forward.h

@@ -74,6 +74,7 @@ class FontsChangeEvent;
 class Toolbar;
 class ToolbarContainer;
 class TreeView;
+class ValueSlider;
 class Variant;
 class VerticalBoxLayout;
 class VerticalSlider;