Add new API functions to get/set table column widths.

[szanni]

Proposal to add API functions to get and set table column widths:

int uiTableColumnWidth(uiTable *t, int column);
void uiTableColumnSetWidth(uiTable *t, int column, int width);

Same as in #513 we need a way to identify columns, so I cherry picked the commit that does column identification by index.

Implementations provided for all platforms (darwin, unix, windows) with a testbed in page16.