CURA-5536 Add the icon indicating when a plugin is already installed.

plugins/Toolbox/resources/images/installed_check.svg

@@ -0,0 +1,8 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Generator: Adobe Illustrator 22.1.0, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
+<svg version="1.1" id="Layer_3" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
+	 viewBox="0 0 512 512" style="enable-background:new 0 0 512 512;" xml:space="preserve">
+<path d="M0,512h512V0L0,512z M440.4,318.3L331.2,431.6c-1.4,1.4-2.7,2-4.8,2c-2,0-3.4-0.7-4.8-2l-53.3-57.3l-1.4-2
+	c-1.4-1.4-2-3.4-2-4.8c0-1.4,0.7-3.4,2-4.8l9.6-9.6c2.7-2.7,6.8-2.7,9.6,0l0.7,0.7l37.6,40.2c1.4,1.4,3.4,1.4,4.8,0l91.4-94.9h0.7
+	c2.7-2.7,6.8-2.7,9.6,0l9.5,9.6C443.1,311.5,443.1,315.6,440.4,318.3z"/>
+</svg>

plugins/Toolbox/resources/qml/ToolboxDownloadsGridTile.qml

@@ -40,6 +40,21 @@ Item
                 source: model.icon_url || "../images/logobot.svg"
                 mipmap: true
             }
+            UM.RecolorImage
+            {
+                width: (parent.width * 0.4) | 0
+                height: (parent.height * 0.4) | 0
+                anchors
+                {
+                    bottom: parent.bottom
+                    right: parent.right
+                }
+                sourceSize.width: width
+                sourceSize.height: height
+                visible: toolbox.isInstalled(model.id)
+                color: UM.Theme.getColor("primary")
+                source: "../images/installed_check.svg"
+            }
         }
         Column
         {