button: Hardcode min-width property set to zero

Buttons come with an intrinsic min-width but lack a method to alter this
property. Setting the requested size to zero has also no effect on it.
The only way found to work is to hard code the CSS into the button.

include/AButton.hpp

@@ -2,6 +2,7 @@
 
 #include <glibmm/markup.h>
 #include <gtkmm/button.h>
+#include <gtkmm/cssprovider.h>
 #include <gtkmm/label.h>
 #include <json/json.h>
 

src/AButton.cpp

@@ -18,6 +18,12 @@ AButton::AButton(const Json::Value& config, const std::string& name, const std::
       default_format_(format_) {
   button_.set_name(name);
   button_.set_relief(Gtk::RELIEF_NONE);
+
+  /* https://github.com/Alexays/Waybar/issues/1731 */
+  auto css = Gtk::CssProvider::create();
+  css->load_from_data("button { min-width: 0; }");
+  button_.get_style_context()->add_provider(css, GTK_STYLE_PROVIDER_PRIORITY_USER);
+
   if (!id.empty()) {
     button_.get_style_context()->add_class(id);
   }