Milky/gui state

[milkmanmatty]

Track which type of tab is selected in the sidebarTabs of MainForm.

This allows for better tracking of GUI's internal state - which in turns allows for better functionality based on the user's selection. This is a precursor branch for Feature bandbox selection.

There are many ways to do this and it ultimately came down to which variety of tight coupling to take:

1. Record the selected tab index, as the index can be used as a unique identifier. Changing the order of the tabs would then break the functionality. Least complex.
2. Record the selected tab name. Possibility of duplicate tab names, while not immediately breaking - might have some run-time exceptions further down the line. Changing the names of any tabs will break the functionality. Also rather simple.
3. Create a custom extended TabPage that can have a readonly property set at constructor time that holds the tab type. Possibly need a custom TabControl and TabPageCollection to ensure existing tab functionality is unchanged. Adding a new tab would break the functionality. Most complex.

This PR implements option 2.