SettingsDialog: fix compiler warning
SettingsPage: remove Obsolete attribute from _loadingSettings, add property IsLoadingSettings.
ChecklistSettingsPage: fix compiler warning
ChecklistSettingsPage: remove obsolete property ChecklistSettingsPage and fix depending code
PluginSettingsPage: fix compiler warning
FormSettings: move the Checklist SettingsPage one level up.

The Checklist is now shown when the "Git Extensions" root node is selected.
(Before, selecting "Git Extensions" caused the treeview to "snap" to the first subnode)
FormSettings: add a Page with introductory text as the Plugin Settings root.
Merge pull request #1683 from feinstaub/topic_settings_dlg

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