Originally, each keyboard devices could have their own keymap.
The one keyboard's keymap could different with others. But for this, Ecore_Drm compile a new keymap when a keyboard is connected. But this is a burden for some people who doesn't manage keymap for each keyboard. They want to maintain only one keymap for system. So, I added cached context/keymap and just ref/unref for each keyboard device. People who want to different keymap for each keyboard just do not set cached context/keymap. Then Ecore_Drm maintain keymaps about each keyboard devices.