button: Add AButton class

The AButton class is designed as full a substitute to ALabel. The
GtkButton attribute 'button_' is initialized with a label. This
label can the be referenced by the subsequent inheritors of AButton
instead of the GtkLabel attribute 'label_' of ALabel.
For convenience a GtkLabel* 'label_' attribute is added to AButton.

If the button cannot be clicked it is disabled, effectively acting
like its label predecessor.

GtkButton seems to catch one-click mouse events regardless of the
flags set on it. Therefore, 'signal_pressed' is connected to a
function creating a fake GdkEventButton* and calling 'handleToggle'
(for details on this possible bug in GTK see:
https://stackoverflow.com/questions/45334911 )

In accordance with other GtkButtons (i.e. the sway/workspace ones)
set_relief(Gtk::RELIEF_NONE) is called on the 'button_' instance.

include/modules/mpd/mpd.hpp

@@ -7,12 +7,12 @@
 #include <condition_variable>
 #include <thread>
 
-#include "ALabel.hpp"
+#include "AButton.hpp"
 #include "modules/mpd/state.hpp"
 
 namespace waybar::modules {
 
-class MPD : public ALabel {
+class MPD : public AButton {
   friend class detail::Context;
 
   // State machine

include/modules/mpd/state.hpp

@@ -7,7 +7,7 @@
 #include <condition_variable>
 #include <thread>
 
-#include "ALabel.hpp"
+#include "AButton.hpp"
 
 namespace waybar::modules {
 class MPD;