move class floating_label from font.cpp to its own file

floating_label is essentially a primitive GUI1 widget, consisting
just of a text label and non-interacting. It's also used for the
map labels displayed in-game iiuc.

src/CMakeLists.txt

@@ -1044,6 +1044,7 @@ set(libwesnoth-game_STAT_SRC
 	display.cpp
 	display_context.cpp
 	events.cpp
+	floating_label.cpp
 	font.cpp
 	format_time_summary.cpp
   	generators/cave_map_generator.cpp

src/SConscript

@@ -80,6 +80,7 @@ libwesnoth_sources = Split("""
     display.cpp
     display_context.cpp
     events.cpp
+    floating_label.cpp
     format_time_summary.cpp
     generic_event.cpp
     hotkey/hotkey_item.cpp

src/display_chat_manager.cpp

@@ -17,7 +17,7 @@
 
 #include "desktop/notifications.hpp"
 #include "display.hpp"
-#include "font.hpp"
+#include "floating_label.hpp"
 #include "game_board.hpp" // <-- only needed for is_observer()
 #include "game_preferences.hpp"
 #include "log.hpp"