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/battery.hpp

@@ -13,7 +13,7 @@
 #include <string>
 #include <vector>
 
-#include "ALabel.hpp"
+#include "AButton.hpp"
 #include "util/sleeper_thread.hpp"
 
 namespace waybar::modules {
@@ -24,7 +24,7 @@ namespace fs = std::experimental::filesystem;
 namespace fs = std::filesystem;
 #endif
 
-class Battery : public ALabel {
+class Battery : public AButton {
  public:
   Battery(const std::string&, const Json::Value&);
   ~Battery();