Add tool modes to the `PathwayNetwork` toolbar

[YuriSizov]

Current toolbar only allows clearing the network (should probably be replaced with an icon button). Tool modes can only be activated by pressing and holding specific keys on the keyboard. This may not always be handy, plus this means that we lack feature parity with the built-in path editor.

So tool mode switches must be introduced to the toolbar. That would be dedicated buttons to select a mode for something that is only currently possible with holding a Ctrl or a Shift. That would include adding and removing nodes; connecting, splitting, and separating paths; adjusting smoothing, including making the curves ins and outs align in a pattern. Most of those, if not all, could benefit from a dedicated tool mode.