Don't move line origin if Alt is held at start

Carsten Hartenfelsrequested to merge
hartenfels/krita:feature/altline into master

Because it's pretty useless to move an empty line, you can just move your cursor, and it interferes with users binding Alt to the quick switch line tool.

Originally reported at https://krita-artists.org/t/how-to-use-alt-left-mouse-click-to-activate-temporary-line-tool/182659

Test Plan

  • Switch to the line tool, hold Alt, start making a line. Observe that it makes a line and doesn't just move the origin of "nothing" around.
  • Lift the Alt key while still making a line. Observe that pressing it again now lets you move the line around.
  • Assign Alt+Left Click to the line tool canvas input and observe that that also works properly now.

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.
  • Does the patch add a user-visible feature? If yes, is there a documentation MR ready for it at Krita Documentation Repository?

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