Allow inserting new source at the position of cursor

[glopesdev]

Currently new sources are always created at the end of the workflow, in a new connected component. Given that subscription order to independent observable branches is determined by top-to-bottom ordering of components, it can often be important to control where exactly new sources get created.

One approach to this is to simply use the current position of the cursor. Similar to how new operators are inserted following the currently selected node, new sources could be inserted in the connected component immediately following the cursor position (or preceding, if shift key is used). Potentially this can be extended for all operators when they are inserted in the workflow without any node being selected, which will be helpful to connect locally proximal nodes in case of accidental insertion.

[glopesdev]

Also related to #1129 where ungrouping is simply interpreted as inserting a node at the same position of the original one.