Change check behavior for Destinations->Image uploader->File uploader

Hi,
currently if you check the menu item "Destinations->Image uploader->Imgur" then "Destinations->Image uploader->File uploader->Dropbox" remains checked. I would say that this is not user friendly. Especially for new users.
I would expect only one menu item to be checked at a time. Either Imgur or Dropbox.

When new user installs ShareX for sharing screenshots using Dropbox, he configures Dropbox as it is suggested in https://github.com/ShareX/ShareX/wiki/Dropbox .
But ShareX still uploads to Imgur. User navigates to Destinations->Image uploader->File uploader, finds that the Dropbox menu item is checked. And it is unclear that he has to either click on Destinations->Image uploader->File uploader or to click on already checked Dropbox menu item.

I suggest to fix it in the following way: 
- If user clicks on a menu item inside Destinations->Image uploader other than 'File uploader' then everything inside 'File uploader' is automatically unchecked.
- If user clicks on Destinations->Image uploader->File uploader then automatically check the item that was last used.

The same issue for Destination->Text uploader.