Replace all margin-left and margin-right with margin-start and margin-end

https://bugzilla.gnome.org/show_bug.cgi?id=710238
GtkPlacesSidebar: don't unref an object we don't own

gtk_settings_get_default() doesn't return a reference, so don't unref it
during dispose.
stylecontext: Revert part of previous commit

Commit 719dd636a9da679ea08a9644a8cccfe7d6f145d7 replaces
margin-left/right with margin-start/end. CSS does not have
margin-start/margin-end properties, the sed script was a bit overeager.

Fwiw, CSS implements RTL margin styling via :dir(rtl) selectors.
GtkSettings: make show-desktop default to FALSE

This broke tests.
Quartz: Set the shows-desktop platform setting

OS X shows icons on the desktop, so set this setting
to TRUE here.
Win32: Set the shows-desktop platform setting

Win32 shows icons on the desktop, so set shows-desktop
to TRUE here.