Use the EWMH support window rather than the root window as an input focus fallback.

Use the EWMH support window rather than the root window as an input focus fallback.

If no other window is available on the active workspace, we now select the EWMH support window (used to indicate that an EWMH-compliant window manager is preent) as the focus window rather than the root window. The NET_WM_ACTIVE window will still be set to XCB_WINDOW_NONE to pretend that no window is actually focused.
This fixes the issue that when using the root window, a fallback mechanism in X11 takes effect which routes keyboard input to the window under the cursor, independent of whether that window has the input focus. Using the EWMH window instead, we can avoid this behavior. We cannot simply set it to XCB_WINDOW_NONE as this would discard all keyboard events, breaking keybindings.

fixes #1378