SelectiveSyncDialog: Change 'partial' behavior.

The real bug in #2219 actually was that the gui allowed users to
choose to not sync the root folder. That's not supported and no
longer possible in the UI.

Instead you may now sync a folder's files without syncing its sub-
folders.
Updater: Add "didn't check for updates" message. #2175

Don't show "Checking server..." when we are not.

(cherry picked from commit ece11dc3d8c906dddf45987cb636e725c5a138fa)