AIconLabel: make regex static and initialize label_contains_icon

Compile the icon-extraction std::regex objects once via static locals
instead of rebuilding them on every update() of every AIconLabel-derived
module (custom, mpris, ...). Also default-initialize label_contains_icon
to false at its declaration.

include/AIconLabel.hpp

@@ -21,7 +21,7 @@ class AIconLabel : public ALabel {
   Gtk::Box box_;
   unsigned app_icon_size_{24};
 
-  bool label_contains_icon;
+  bool label_contains_icon{false};
 
   bool iconEnabled() const;
 };