This includes some not-so-nice lifetime handling of the button
"clicked" connection handling. I decided to keep the code in
one place, simply always forcing a disconnect on show, instead
of a more "optimized" code version in SetFrame.

First we try to get the icon from the system theme, but use LO's
own icon theme as a fallback.

Change-Id: Ic0459623ec907b9a54bef4670bf65cf587cd47ea
Reviewed-on: https://gerrit.libreoffice.org/71784Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Jan-Marek Glogowski <glogow@fbihome.de>