kcms/keys: Add a separate "Add Command" button

After reading a good amount of feedback and questions from users, it seems that it is not clear that the textbox in the "Add Application…" dialog can be used to add custom commands.

Some examples of user confusion that I found:

• https://old.reddit.com/r/kde/comments/xw8k06/the_new_shortcut_page_in_wayland_is_a_big/
• https://old.reddit.com/r/kde/comments/x0tmr5/custom_keyboard_shortcut_to_run_command/
• https://old.reddit.com/r/kde/comments/xf59gs/how_do_i_create_a_custom_shortcut/
• https://old.reddit.com/r/kde/comments/xf4a3g/global_mappings/
• https://www.reddit.com/r/kde/comments/xe59pe/custom_shortcuts_disappeared_in_system_settings/

This MR adds a separate button to add a command, which right now opens essentially the same dialog but with a "Type in a command:" message.

This still feels like not quite enough to solve the confusion (mainly the app list still showing in the Add Command dialog etc), so I'm looking for feedback and suggestions of other approaches from VDG and Usability.

cc @teams/vdg @teams/usability