GUI: Removed ability for a middle state to be a start/end state

[jslane-h]

Straying a little away from the original intent of the ticket, this PR makes a user unable to change a middle state to a start/end state. The reason behind this is mostly because the GUI cannot support multiple root nodes (maybe we'd want this in the future?), and changing a middle state to a start/end state introduces an extra root node.

List of Changes:

• Removed ability to change a middle state to a start/end state
• Removed ability to set a "Choice"-type state to a start/end state
• Refactored so that difference between advanced state forms, basic state forms, and general state forms is more obvious throughout the code
• Removed logic of deleting a state out of the state item as a separation of concerns (states now signal to the workflow diagram that they are going to be deleted)
• Added a context menu event so that you can right click a state and select edit instead of only allowing double click

[leijerry888]

Hi @jslane-h , could you please merge develop in and resolve the conflicts