CURA-4578 Manually installed plugins handled correctly

3rd-party plugins which were manually installed were listed as uninstall-able instead of disable-able. The distinction between plugins which could be installed or disabled was previously made by whether or not it was 3rd party, but is now made by determining the install location.

plugins/PluginBrowser/PluginEntry.qml

@@ -164,7 +164,7 @@ Component {
             Button {
                 id: removeButton
                 text: "Uninstall"
-                visible: model.external && model.status == "installed"
+                visible: model.can_uninstall && model.status == "installed"
                 enabled: !manager.isDownloading
                 style: ButtonStyle {
                     background: Rectangle {