Context menu, delete, duplicate, and pin function

[DoomTas3r]

Adds a context menu with 4 functions: duplicate, summary, pin, and delete

Adds a duplicate function with ctrl + d as a shortcut as suggested by #201 (comment) and #267 (comment). The offset is 100 pixels right, 50 pixels down. The snapped blocks and parameter values are also duplicated

The summary shows the tooltip of the block, intended to be replaced by a more detailed overview like #291 (comment)

Adds a pin function with ctrl + p as a shortcut. Pinned blocks can't be dragged. Snapped blocks can pin with their parent block. This feature doesn't need to be added because the undo feature can undo the effects of accidental drags or deletes, so this is just for convenience

_confirm_delete is its own function

Functions that check snapped blocks only check Container nodes because only they have SnapPoint nodes

When the drag drop area receives a right click, a new popup menu determines which function is called from the parent block

[manuq]

@DoomTas3r thanks! There is a bounty for the duplicate functionality. Please follow the steps in #201 (comment) if you would like to attempt it.

[DoomTas3r]

Thanks, but I will not accept or claim the bounty

[manuq]

@DoomTas3r I see that you are trying to add pin functionality to this PR as well, which was never discussed. My suggestion is to try contributing smaller chunks, because they are more likely to be merged. For example I would start with a context menu and just the current Delete functionality that is already there (as Del key shortcut).