Issue #10875: Redesign Welcome Dialogue options layout

This patch makes a fairly small redesign to the Personalize section of the Welcome Dialogue to help with #10875 (closed).

The original design I made had the "Merge Titlebar" and "Check for updates" options on the same row. As @Wormnest noted, depending on translations this might cause the dialogue to expand horizontally.

I converted the three Additional Customization options to all be GtkSwitch and then lined them up vertically as seen below:

@Jehan, @Wormnest, @brunolopesdsilv, @rangelovd, @hopamalopa, please let me know what you think when you have time. It's not a full redesign, but hopefully it's a small improvement over the current layout.

Revision 1