Explore Possibility of using tksheet for main activity window

[aussig]

Currently we manually create loads of individual UI elements for each system tab in activity windows, which works well but has a significant overhead.

We're already using tksheet to manage Discord webhooks in the preferences, and it might be neater, more flexible, more efficient and give more controls to the user if we switch to using a tksheet for the activity table, but only if this keeps the clarity of display and ease of use.

Experiment.

[aussig]

Downside: tksheet uses tkinter as its base, not ttk. This will mean the future theming rework of EDMC will not automatically apply to this component. Note that we will already have this problem, as it's used in the settings window.