Gumpu/stop assistant creation with escape key

Added the possibility to interrupt the creation of a new assistant by pressing the escape key.

Test Plan

Steps:

• Select the assistant tool.
• Select one of the assistants that has more than one handle.
• Start creating the assistant.
• Press Escape

Result: The half finished assistant disappears.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.

Note: seven unittests failed but they were in unrelated modules.