Add an 'Add Command' button to the Shortcuts KCM

Replacement for !1189 (closed). This time a separate dialog which doesn't involve KIO.

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 opens an overlay sheet:

cc @teams/vdg @teams/usability

BUG: 460036 BUG: 440431 FIXED-IN: 5.27