exo: Improve wl_shell_surface configure events.
exo: Improve wl_shell_surface configure events.

Implement a widget observer that will send configure events
if the size of maximized or fullscreen shell surfaces change.

BUG=549781
TEST=exo_unittests

Review URL: https://codereview.chromium.org/1849783003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#385629}

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