Add labels to nodes & connectors

[MattIPv4]

Currently, a user cannot add a label to a node to describe what its purpose is. Further, a user cannot add labels to connectors, to describe the data flow etc.

We should allow the user to add a label to a node. We should provide an option for the position of the label: below the node, above the node, to the right of the node, to the left of the node.

Similar to the node, we should also allow the user to add a label to a connector. For positioning of this, we should provide two dropdowns that combine to define a position. The first should be an option to put the label either above or below the line. The second dropdown will then offer three options for where along the line the label should be: start, middle, end.

(I am unsure how to handle vertical lines, the above is written for horizontal lines. I assume we should have the text always be horizontal, so maybe we need logic to switch top/bottom to left/right if the line fits certain criteria making it "vertical" instead of "horizontal"?)

These label options for both the nodes & connectors can likely be offered in the panel that is shown when a node is selected. If not already implemented, we should add similar selecting & UI panels for connectors, which can then contain the label options.

Whilst we are adding new labelling, we should also consider improving the label options for a category. We can likely do a similar thing to the above, where we add the ability for a category to be selected, and then show the UI panel where the user can edit the label text and positioning. When we get to this point we can discuss further the positioning options that we offer (top/left/right/bottom + corners, all inside & outside the category boundary?).