Add new API function to set a table row double click callback.

[szanni]

Proposal to add a new API function to set a callback for when a user double clicks a table row.

void uiTableOnRowDoubleClicked(uiTable *t, void (*f)(uiTable *t, int row, void *data), void *data);

Implementations provided for darwin, unix, and windows. A small sample usage is provided in page16.

Only thing I'm not quite sure about is the naming. Alternatives would be uiTableRowOnDoubleClicked or instead of DoubleClicked use Activated if we should ever want to support single click activation and disable row selection?