Tweaked AddCombatant and RollInitiative

User must indicate whether they want to use the HP balancing formula
when adding batches of combatants. RollInitiative won't open unless
there's at least one combatant in the combatant list (if the user tries
to open it then an error dialog appears).
Added support for different die type in batch add

Some monsters/characters have complex HP expressions. For example, a
multiclass fighter/rogue might have an HP expression like 3d10+4+2d6+4.
The new version of batch add can now calculate this. Also the user can
only check/uncheck "use generous HP" when they are using batch add.

Made DM tab the default tab for General, because I use it way too much
and every second counts when your players are entering a fight to the
death!