[baseIcon] Tie the icon's width to its height

[baseIcon] Tie the icon's width to its height

Both current design and new mockups use square icons. Currently this
is implemented by setting a fixed width and height in the CSS, which
does not work for the growing/shrinking items in the new sidebar.
By moving the assumption of square items into the code, the size will
adjust according to icon size, spacing, padding and font size if not
explicitly overwritten in the CSS.

https://bugzilla.gnome.org/show_bug.cgi?id=625887