Change SVG source zoom level only when on SVG source tab

Previously you could Ctrl+Wheel on Rich Text tab, and be able to change zoom level in SVG source tab.

Test Plan

• Open a text object
• Switch to SVG source tab
• Remember the zoom level
• Switch to Rich Text tab
• Scroll the mouse wheel while holding Ctrl
• Switch back to SVG source
• Retain the previous zoom level
• Scroll the mouse wheel while holding Ctrl
• Have the zoom level change

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.