Improve close window icon

## Prerequisites
- [X] I have searched open and closed issues for duplicates.

## Feature
**Is your feature request related to a problem? Please describe.**

I think the close window icon is too small for Elementary window headerbar. Other icons in the window headerbar are larger and looks a little bit weird. I don't see that problem happening with maximize button though.
I notice that the close icon has the same size that GNOME's. However, Elementary's headerbar uses buttons without borders, which is different from GNOME approach. The latter, because of the borders, seems a different button or action, so looks consistent, while the former looks like a papercut in the icon design.

* Elementary
 
* GNOME

**Describe the solution you'd like**

I'm not a designer, so I might not have the knowledge to suggest a proper solution. What I could suggest are two options:
1. Increase the icon size. Make it the same size of the other icons in the headerbar. I think the size of window buttons vs window title size in Windows is a good ratio example:
2. Make other buttons in headerbar to have borders like GNOME's. Not sure if this will work because it'll add more constraints to what can fit in the button borders (check example above).

**Existing work**

I didn't find anything/anybody working on this in Elementary.

**Describe alternatives you've considered**

Just use what we have today :).

**Additional context**

No, I'm good, thanks.

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