KeyboardMapper: Extract method map_from_name

Extract the mapping of a name to a character map into its own method.
This only slightly reduces the number of lines, going from 24 to 17
lines, but makes the code somewhat more readable and reduces repetition.

Userland/Applications/KeyboardMapper/KeyboardMapperWidget.h

@@ -35,6 +35,7 @@ private:
     Vector<KeyButton*> m_keys;
     RefPtr<GUI::Widget> m_map_group;
     void add_map_radio_button(const StringView map_name, const StringView button_text);
+    u32* map_from_name(const StringView map_name);
 
     String m_filename;
     Keyboard::CharacterMapData m_character_map;