Updated tab and pill creation and fixed rendering

[kuro337]

@akinsho

Hey Akin, apologies sir - the PR was quite big - I will make an incremental PR.

This was the main update - for the tab and pill rendering creation for the groups.

All the changes are in the groups file alone now -

1. Fixed/enhanced the separator.tab and separator.pill logic (lines 47-157)
2. Fixed the render function (lines 628:700)
3. Added a function to remove a buffer from the group to have a more flexible grouping approach.

Let me know if there is any discussion to be had - I tried adding descriptive comments everywhere!