Apply some CSS goodness to the Cylinders widget

Alternating color and some hover highlighting to make things prettier.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix colum visibility selection in the divelist

Several changes:
- split the reload of the DiveListView from the reload of the header
- don't include the column title in the name of the setting; the title
  will change depending on the units and localization chosen by the user
- rename the slot that toggles visibility to make the code more readable
- use setCollumHidden() method to simplify the code
- don't save the width of hidden columns (as they would be saved as zero
  width and can then no longer be enabled)

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Repair the saving and loading of units

This way it should work...

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Hook up most of the Preferences dialog

The imperial/metric super setting doesn't have any effect. But changing
the individual units now works and is tracked. And causes the display to
change after clicking "OK" (but not yet when clicking "Apply").

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>