Improvement. UI. Combine Cancel button with Execute button.

[podkolzzzin]

Background:
There is no way when both of this buttons would be enabled.
So:

• When nothing is executed show Execute button
• During Execution show 'Cancel' button

It would be useful to have 'Fork' button when execution is started. It can be on the right part of 'Execute'/Cancel' button.

[podkolzzzin]

Fork button should open a new tab with the same settings as current