new tool: gui/pregnancy

[myk002]

productionization of #873

changes:

• UI now dynamically enables valid widgets, e.g. disabling UI elements that don't make sense in the current state
• if the same unit is named multiple times in the UI, the name will have the same color in all places
• business logic significantly simplified
• window appears off to the side so the player can more easily select units as the parents
• automatically load the spouse if no other partner has been chosen yet
• show whether the currently selected female is pregnant (and how long it will be before she comes to term)
• redo layout using dividers for a cleaner look

[myk002]

demo: