Add American-style preparation time

[czlee]

Suggested approach: The main counter always represents the total prep time elapsed (including all stages), and the prep time used in this stage is a new set of methods. The brainwork is all done in DebateManager, i.e. DebatePhaseManager doesn't require modification.

• DebateManager#resetActivePhase() would discriminate prep times apart and not call DebatePhaseManager#reset() if it is a distributed prep time phase, but instead just set stop the timer and set the current time to the total prep time allowed less the sum of all other stages.
• Prep time phases are specified by <prep team="affirmative"> in the XML file, and as far as the app is concerned are just numbered from 0.
• Would need to override the period info description with the name of the next speaker.
• Would need new member mPrepTImes being a map of enums (for the teams) to a list of times.
• You can always calculate the time for this phase in DebateManager by subtracting the sum of all other phases from the time elapsed in this phase.
• Time elapsed in this phase is shown in smaller font where the POI button normally is, and always counts up, never down as that would make no sense.
• No need to worry about length/overtime from GUI perspective, as these don't change. What changes is the initial value of the timer, as it is set to the time remaining of all prep time stages.
• However, will need to update controls so that "Start" is displayed when the time remaining is equal to the total time available minus the time used in other prep time stages.

Then the phase index logic will have to be written to accommodation for distributed prep time.

[czlee]

Superseded by czlee/debatekeeper#6