Calc: Borders setting via Notebookbar overwrites existing border

[thebearon]

If you set various borders via the Borders dropdown widget in Calc, the current setting will overwrite the previous one. This isn't the expectation, the no border setting should clear all borders, but all the other settings should be set in addition to the existing setting.

Eg. if you first set left border and then top border, the expectation is to have both left and top borders in the selected cell/range.
Right now top border would replace the left border.