638403: IsoIconBrowser:" Make it use modern ExtensionSystem::IPlugin interface | https://codereview.qt-project.org/c/qt-creator/tqtc-plugin-qtquickdesigner/+/638403

Moved the plugin to the normal Qt Creator plugin loading mechanism.
Which in principle is a good thing, but if the plugin is enabled by default it will force the QmlDesigner plugin to be enabled too, even though that should be disabled by default. And if the IsoIconBrowser plugin is disabled by default, it is a bit unexpected that it stays so if the QmlDesigner plugin is manually enabled by the user.

643703: Disable by default for now | https://codereview.qt-project.org/c/qt-creator/tqtc-plugin-qtquickdesigner/+/643703

Works around the automatic loading of QmlDesigner, but leads to the second issue.