Divelist: remove units from header, add tooltip

This should help with the issues we are having with alignment / visibility
of the relevant information in the divelist.

Most people will never switch the units (or maybe switch them once, the
first time they use Subsurface). So having the units shown in the table
header quickly becomes redundant - yet having the units in there causes
all kinds of layout issues.

This patch adds a tooltip to both the header and the main data area of the
divelist. The tooltip shows the column name plus unit and the column
header no longer shows the units.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Remove debug output in preference setting

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Added close button to print preview window title bar.

Without this I was only able to close it by choosing to print.

Signed-off-by: John Van Ostrand <john@vanostrand.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>