[FIX] web: fix TAB key in editable list views

Before this commit, two variables were used to store the current
position in a list view: currentCol and currentRow. This made it
difficult to handle incremental moves (eg: tab, shift+tab) inside a
row, since we have to ignore all buttons. Previously, this was done by
converting 'currentCol' to 'currentFieldIndex', but the conversion
between the two variables was incorrect and does not appear to be
that easy to fix. As an example, it was not possible to TAB-navigate to
the last column of the components list when creating a BoM.

This commit removes all mentions of 'currentCol' and uses
'currentFieldIndex' instead, removing the need for conversions. The TAB
navigation code is simpler as a result, and should now be correct.