In Calc, the background color toolbar behaves differently than the identical
toolbar button in Writer.

Steps to reproduce:
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1. Select some cells in any spreadsheet.

2. Click the background color toolbar button.  The color palette appears and can
be used to pick a background color. The selected cells acquire the color you chose.

3. Select a different set of cells in the spreadsheet.  The toolbar button
changes to reflect the color of those newly selected cells, instead of retaining
the highlight color that you had previously chosen.

4. Now, to highlight the second set of cells with your preferred color, you have
to re-select the exact color you chose previously from the palette.

What should happen:
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As in both Writer and MS Excel, the background color toolbar button in Calc
should have two UI components:  the square of color, and the dropdown arrow.  

When the user clicks on the square of color, that color should be applied to the
currently selected cells.

When the user clicks on the dropdown arrow, (1) the palette should appear so
that the highlight color can be chosen, and (2) that color should be applied to
the current selection.

Changing the cell selection should never change the toolbar highlight color.