Improvements to Null values handling

[dferrand]

Multiple changes to null value handling:

• In addition to being in italic, a null value now has a highlighted background (as suggested by @mipjones)
• When a field is updated from a null to a non null value, the italic and highlighted background is now removed (fixes UX: update mode output formatting does not refresh when _null_ value is changed #403)
• Setting a field to null is now done by pressing shift+enter instead of entering null (or - for a field shorter than 4 characters). A tooltip indicates this information if the field is nullable. This allows setting a field to the "null"string (or "-" if shorter than 4 characters). (fixes UX: No obvious way to enter null value when in update mode #402)

[worksofliam]

The functionality is excellent! I love the Shift+Enter notice. Here are a couple of things I noticed:

Looks like some css issues with the columns:

When applying null, the font is not set to monospace as the other columns are, though it's not consistant:

[dferrand]

I made a few more changes:

• Fixed the css issues
• fixed the font
• column widths on result set are now adjustable. If you double click on the width adjustement handle, it will auto adjust the width to the content
• You can now choose to display both column name and label in the header