ecore_wayland: Add a new API to get keymap - ecore_wl_input_keymap_get()

Summary:
Each Ecore_Wl_Input structure has there own keymap.

This keymap is struct xkb_keymap used in libxkbcommon lib and
the client can get this keymap from server in wayland system.
So if the client want to get keymap and use some libxkbcommon APIs,
use this API to get keymap.

Test Plan: Use this api after initialize wayland system.

Reviewers: raster, zmike, ManMower, devilhorns

Reviewed By: devilhorns

Subscribers: input.hacker, ohduna, cedric, jpeg

Differential Revision: https://phab.enlightenment.org/D3819