UserListDialog: Fix some layout issues

Remove the content box' border, mark all rows unactivatable and change
all the debug output from g_message to g_debug
MainWindow: Don't access screen_name of null accounts
ModifyFilterDialog: Use header bar if the settings say so
UserListDialog: Use header bar if the settings say so
UserStream: Print json data in case of an exception
AccountDialog: use header bars if the settings say so
ImageCropDialog: use header bar if the settings say so