Pipeline: Use preferred size for min and max for sources window.

Without this, the sources window shrunk when enough processing steps were
added, making it at first hard to read and then impossible to use.

Author: PeterJohnson

ui/src/main/resources/edu/wpi/grip/ui/pipeline/Pipeline.fxml

@@ -13,7 +13,9 @@
 <StackPane fx:id="root" maxHeight="Infinity" maxWidth="Infinity" styleClass="pipeline" xmlns="http://javafx.com/javafx/8.0.40" xmlns:fx="http://javafx.com/fxml/1" fx:controller="edu.wpi.grip.ui.pipeline.PipelineController">
     <children>
         <HBox maxWidth="1.7976931348623157E308">
-            <VBox fillWidth="true" maxWidth="1.7976931348623157E308">
+            <VBox>
+                <minWidth><Control fx:constant="USE_PREF_SIZE" /></minWidth>
+                <maxWidth><Control fx:constant="USE_PREF_SIZE" /></maxWidth>
                 <Label maxWidth="Infinity" styleClass="pane-title" text="Sources" />
                 <Separator orientation="HORIZONTAL" />
                 <Pane fx:id="addSourcePane" />