Code cleanup and check for OOM condition with wl_array_add

Code cleanup and check for OOM condition with wl_array_add

Summary:
wayland: Check wl_array_add return for OOM condition
cosmetic: Make error return check syntax consistent with rest of code
wayland: Reuse use_adj to track child-forced adjust
wayland: Refactor to collapse if chain
wayland: Refactor _e_comp_wl_client_priority_adjust to collapse if chain
 + This uses early returns to unwrap a couple if clauses to make the code a bit less nesty.
wayland: Refactor if statements for consistency with rest of file

Reviewers: cedric, zmike, devilhorns

Subscribers: devilhorns, cedric

Differential Revision: https://phab.enlightenment.org/D2034