LibKeyboard: Mark CharacterMap::get_char as const

Also, mark character_map_name method as const and make it to return
const String& instead of const String.

Userland/Libraries/LibKeyboard/CharacterMap.cpp

@@ -74,7 +74,7 @@ Result<CharacterMap, OSError> CharacterMap::fetch_system_map()
 
 #endif
 
-u32 CharacterMap::get_char(KeyEvent event)
+u32 CharacterMap::get_char(KeyEvent event) const
 {
     auto modifiers = event.modifiers();
     auto index = event.scancode & 0xFF; // Index is last byte of scan code.
@@ -121,7 +121,7 @@ void CharacterMap::set_character_map_name(const String& character_map_name)
     m_character_map_name = character_map_name;
 }
 
-const String CharacterMap::character_map_name()
+const String& CharacterMap::character_map_name() const
 {
     return m_character_map_name;
 }

Userland/Libraries/LibKeyboard/CharacterMap.h

@@ -47,12 +47,12 @@ public:
     static Result<CharacterMap, OSError> fetch_system_map();
 #endif
 
-    u32 get_char(KeyEvent);
+    u32 get_char(KeyEvent) const;
     void set_character_map_data(CharacterMapData character_map_data);
     void set_character_map_name(const String& character_map_name);
 
     const CharacterMapData& character_map_data() const { return m_character_map_data; };
-    const String character_map_name();
+    const String& character_map_name() const;
 
 private:
     CharacterMapData m_character_map_data;