SettingsDialog: fix compiler warnings / put pages to the root nodes

1. Fix compiler warnings and todos left over from the refactoring when the tree view was first introduced.
2. Before this PR if the user tried to collapse the "Git Extensions" or "Plugins" root node the tree expanded automatically and the first subnode was selected. This was not very intuitive and is fixed now by
 - move the Checklist Settings page to the GitExt root node
 - add a SettingsPage with introductory text to the Plugin root node