fix(modal): put backdrop in before window

this compiles and inserts the backdrop before the window object. this change was previously accepted and then reverted by recent changes. putting the backdrop first in the dom allows the natural stacking order to stack the modals. it also gives users (who may be overriding the tpls) the chance to set z index first on the backdrop before setting it on window. this is helpful if you have a global notion of next highest z index.