Finalized most of category manager. Read full description.

* Updated category model delete to update cache
* Added mouse listeners to improve selection in manager
* Made UI layout consistent with rest of project
* Implemented proper refreshing upon addition, edition, and deletion
* Modularized some code
Merge branch 'add_by_category' of https://github.com/cs3733-team4/wpi-suite into add_by_category