Enlarge the UI to support 4k HiDpi monitor

**Description**
the UI is very small, both icons and font do not scale properly when Pinta for Windows is used on 4k or HiDpi monitors. 
Here more details from the GTK project: https://wiki.gnome.org/HowDoI/HiDpi/ 
Below is a screenshot

**Additional context**

![Image](https://github.com/user-attachments/assets/e5aad4fb-1032-436e-a8c5-86670149ed90)
