Update Icons

Subdivided the new icons in 3 size categories. As requested by UX
Used the company naming scheme for uniformity and easy recognition.

Known issues:
- Top/Bottom category wasn't taken into account by UX at the time. Since
 this is a recent addition. Both the Walls and Top/Bottom will be updated.
- Cloud/Network connection icons 12px don't render correctly. Due to the
 theme-ing.
- Extruder Icons do not render correctly.

CURA-8010_new_icons

plugins/PrepareStage/PrepareMenu.qml

@@ -106,7 +106,7 @@ Item
                 {
                     id: buttonIcon
                     anchors.centerIn: parent
-                    source: UM.Theme.getIcon("load")
+                    source: UM.Theme.getIcon("Folder")
                     width: UM.Theme.getSize("button_icon").width
                     height: UM.Theme.getSize("button_icon").height
                     color: UM.Theme.getColor("icon")