Add selection of n-gram groups to type among the listed top 200

[kev-ace]

First of all, thank you very much for making such a useful tool in such a user-friendly manner! You've helped us typists to achieve our goals in a breeze.

Regarding my suggestion in this ticket, currently the "scope" covers the top 200 bi-, tri- and tetra-grams, which is more than plenty to play with. However, the user is unable to have a more targeted practice of a specific portion of the listed n-grams. For example, if you want to practice the last 50 listed n-grams, you have no other way but to select the top 200 and go through all of them to get the last 50 mixed somewhere in it.

Taking this into consideration, I believe it would be beneficial to add at the very least a group selection of which quarter of the list you'd like to practice. This will make it easier for users to target certain portions of the n-grams without needing to have to go through the extra load of the rest of the content of the list.