Stopped empty strings from being added as tags.
Options window is now centred on the screen when opening.
Fixed issue where the distinct in a linq query in TagSelection.InitialiseItems wasn't working because I was selecting a custom data type. Fixed this by implementing IEquatable and checking that the tags are equal.
When the user changes the tags, if the timer is currently running, then it now restarts the timer, so that a new history item is created containing this new tags.