Allow for reward grouping.

This commit makes the MAUtils class responsible for parsing reward maps
use the new ThingPickerManager to parse the reward lists. As a result,
the new `all()` and `random()` pickers are available for use in the
rewards section.

This should allow for granting item sets and similar types of "bundles"
as rewards in the arena, e.g. a diamond sword and permission to join a
more difficult arena.

Closes #386

changelog.md

@@ -18,6 +18,7 @@ These changes will (most likely) be included in the next version.
 - Boss names now support color codes.
 - New per-arena setting `arena-warp-offset` can be used to spread out players randomly by an offset from the arena warp. This should help prevent players taking suffocation damage.
 - New per-arena setting `announcer-type` determines where to display per-arena announcements such as wave spawns, auto start timers, boss abilities, and death messages. Options are `title` (default) or `chat`.
+- It is now possible to group rewards. For example, `all(stick, bone)` results a stick and a bone, while `random(all(stick, bone), all(dirt, stone))` results in getting _either_ a stick and a bone _or_ a dirt block and a stone block.
 - The Root Target ability now uses potion effects (slowness, slow falling, and negative jump boost) instead of repeated teleports. This should make for a smoother root experience.
 - Using `spectate-on-death: true` no longer forces players out to their join location/exit warp before moving them to the spectator area. This should prevent "jumpy" behavior in multi-world setups.
 - Players should now properly respawn at the spectator area rather than at world spawn on servers with plugins that override respawn locations.