Detect conflicts in Canvas Input Settings

The warning when hovered or pressed, shows a tooltip like this:

All conflicted actions get a warning (in this case, Zoom Canvas and Touch Gestures conflict on Four Finger Tap):

Test Plan

1. Try to load configs that have conflicts, check if the warning appears for the conflicting actions.
2. Try to modify the conflicting shortcuts, check if corresponding warning disappears.
3. Try to create conflicts by modifying certain shortcut, test having same shortcut mapped to two different modes of same action and same shortcut mapped to two separate actions. Check if warning appears.

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.