Shifts all clients per tag to leftmost unoccupied tags.
For example, if clients A, B, C are tagged on tags 1, 5, 9 respectively, when this function is called, they will now be on 1, 2, and 3. The focused client will also remain focused.
Clients on multiple tags will be treated as if they only were only on their leftmost tag, and will be reduced to one tag after the operation is complete.
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