Clean up unnecessarily sparse z indices
Fix label text field view to stay on top again

SC update removed zIndex:100 from text field views
rm deprecated TextFieldView property
rm obsolete fix for paste event in TextFieldView

SC TextFieldViews now update value property on cut/paste.
rm obsolete fix for placeholder feature detection

SC.platform.input now does placeholder feature detection, and TextFieldViews now do the right thing when placeholder support is detected (or not).