desktop-shell: shell: Capture input on fade animation curtain

Shell fades out and in depending on user interactions. When screen is faded out, first interaction wakes up the shell from idle and screen fades back in. However, this first interaction is applied to whatever is "behind" faded screen and it may trigger unwanted actions (first touch or mouse click on active GUI elements). Behavior is the same for both idle and lock screen (because unlock dialog is created after the first interaction).

Update fade animation curtain creation function to capture inputs in order to prevent unwanted interactions within faded or locked shell.

Fixes issues #147 (closed) and #569 (closed).

Signed-off-by: Witold Lipieta witold.lipieta@thaumatec.com