Improve usability of Sign API (#1202)

* Improve usability of Sign API

* PHP

* Throw exceptions if out of range

* Fix phpdocs

* Formatting, additional checks in the SignChangeEvent.

* Blame php storm

* require line count to always be 4

* Adjust exception message

src/pocketmine/tile/Sign.php

@@ -64,6 +64,33 @@ class Sign extends Spawnable{
 
 		return true;
 	}
+	
+	/**
+	 * @param int    $index 0-3
+	 * @param string $line
+	 * @param bool   $update
+	 */
+	public function setLine(int $index, string $line, bool $update = true){
+		if($index < 0 or $index > 3){
+			throw new \InvalidArgumentException("Index must be in the range 0-3!");
+		}
+		$this->namedtag["Text" . ($index + 1)] = $line;
+		if($update){
+			$this->onChanged();
+		}
+	}
+	
+	/**
+	 * @param int $index 0-3
+	 *
+	 * @return string
+	 */
+	public function getLine(int $index) : string{
+		if($index < 0 or $index > 3){
+			throw new \InvalidArgumentException("Index must be in the range 0-3!");
+		}
+		return (string) $this->namedtag["Text" . ($index + 1)];
+	}
 
 	public function getText(){
 		return [