TANGO: improve existing icons used in calc's toolbar and provide sources

Change-Id: I03ea3037f40bd1a0c30dff456df3fe75978c44bd
Reviewed-on: https://gerrit.libreoffice.org/12418
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>