TenBrushes: Add an option to automatically select freehand brush tool when using a brush shortcut

This adds an option to the TenBrushes plugin, enabling the automatic activation of the freehand brush tool when using any of the configured brush preset shortcuts. Disabled by default.

Test Plan

Enable TenBrushes plugin, configure presets in TenBrushes, enable this option, select an unrelated tool, and use one of the configured hotkeys to observe the behavior.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work. (I wasn't able to test the specific behavior this modification allows in master when I tested it several hours ago, but the errors are likely unrelated to the plugin modifications as I've been running this for months in stable Krita.)
• 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.