gtk3: hive the borders and padding calc off into a standalone
gtk3: hive the borders and padding calc off into a standalone

Change-Id: I0890a7d8c50b8646782bfc4358aa7c2f106dc22d

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core