possible future protobufs for handling category channel selection.

Summary:
The initial code for the voting booth.
Some things are still missing:
1. comments EVERYWHERE
2. an implementation for the encryptor (program crashes when trying to encrypt PlaintexBallot)
3. the OutputDevice class should become a thread, (runnable with a queue of commands as the UI component)

Test Plan: Currently only simply run it with another main class.

Reviewers: arbel.peled

Differential Revision: https://proj-cs.idc.ac.il/D3

meerkat-common/src/main/proto/meerkat/voting.proto

@@ -39,6 +39,17 @@ message BallotQuestion {
 }
 
 
+message QuestionCluster {
+    UIElement cluster_description = 1;
+    repeated int32 question_index = 2;
+
+}
+
+message Channel {
+    UIElement channel_description = 1;
+    repeated int32 cluster_index = 2;
+
+}
 
 // An answer to a specific ballot question.
 // The answer is a vector of signed integers,