EMI: Rework text stack handling

The text stack in EMI must be saved and restored in save games because the game will call PopText after returning from the menu. Instead of managing separate lists for each text stack level this patch adds a
_stackLevel field that is incremented for PushText and decremented for PopText. Only text objects from the stack top (level 0) are rendered.

Fixes a false assertion when loading a game from the launcher, when PopText is called without PushText.