[background] cleanup and refactor, fix several issues

Namely:

1. In caja_is_drawing..(), only get atom for WM_CLASS property if it exists.

2. Cache gsettings draw-background & show-desktop-icons key-values, and only update
when either actually changes, to avoid unnecessary queries to the dconf-service.

3. Avoid conflicts between settings_change_event_cb() & on_bg_handling_changed().

4. Keep track of screen sizes, to ensure size actually changed when signalled, to
avoid double-redraw on an extraneous signal, which can lockup the daemon, because
we draw immediately (w/out g_idle_add). Though we don't fade in re-draws triggered
by size/monitors-changed signals, making all re-draws queued causes issues with
fading, when we do fade (on bg change), and queueing is unnecessary here anyway.

5. Track a redraw in progress, to avoid any possible (and unforseeable) clashes.

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