fdo#71748 Fix color pickers

This commit includes the following changes:

1) Remove boxes from PNG files, and draw them ourselves.
This will ensure that the color rec will always fit the box.
Also it will make all color pickers look the same, and greatly
simplify the code. (a part of the code was not reachable anyway.)

2) Fix color preview for toolbar large icons. (a regression of
commit f59c47fc1e79050d3342924e6173b0f4b6ddaf02.)

3) Fix sidebar paragraph color picker initial drawing. reset should
be called *after* the icon assignment.

Change-Id: I13ae02bc3dd68dd14877ccc949784072c58bf211
Reviewed-on: https://gerrit.libreoffice.org/6882
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>