EMI: Fix color of menu entries

The highlighted menu entry is sometimes not correctly colored.
There are multiple textObjects per string (one for each color) and
residualvm draws them in an unspecified order.

To ensure that the correct textObject is drawn on top, each
object gets a layer attribute assigned by the lua code.

The patch adds the layer attribute to the runtime engine and orders the
textObjects by layer before drawing them.
Merge pull request #765 from chkr-private/menu-text-fixes

EMI: Fix color of menu entries