Began attack and damage implementation

On the DM tab the DM can keep track of the combatant's attack and damage
rolls. + and - buttons allow quick changes to the bonuses provided.
Finished implementing attack and damage support

Will look at colouring the HP bar depending on current HP total next.
After that, the delay system. After that, will add functionality to the
"move up" and "move down" buttons (this will require some R&D on the
ordering of items in a collection). Finally I'll implement effects (+1
to all attack rolls for example) along with remaining rounds of effect
that are decremented each time the round number is incremented.
Started implementing HP bar colour update

Need to work out what colour the default is and then use that for
"healthy".