Merge branch 'terencehill/itemstime' into 'master'

Items Time

Features:
* Panel that displays left time until important items respawn in the map: mega health, large armor, powerups (strenght, invincible, jetpack, fuel regenerator) and superweapons.

* Waypoints for mega health and large armor, displayed just like the existing powerup waypoints, obviously visible only to spectators

Panel and waypoints can be enabled when spectating (g_waypointsprite_itemstime 1, hud_panel_itemstime 1) or also when playing in warmup stage (g_waypointsprite_itemstime 2, hud_panel_itemstime 2).

In order to prevent cheating, server can completely disable this feature by setting sv_itemstime to 0 (1 by default).

See merge request !133