Improve Styling / UI

Changes done

• fix main the scrollbar (missing `margin: 0` on body)
• style header (reduce the difference with Studio)
• add background colors to highlight the work area to the other columns
• Put the `nuxeo****layout.html is not customized yet. [CUSTOMIZE] ` sentence in a centered box
• left column: make selected view more visible (bold is not enough)

Remaining

• add to body font-weight: 100; font-size: .9em; to make the UI lighter
• Update NXS-3700 sentence by "Select on the left panel the template you want to adapt. Then, drop new elements or configure properties." (sorry, I found a simpler sentence after your task resolution)
• add a style for these "no result" sentences (to apply to NXS-3699 empty result too)
  font-size: 1.2em; line-height: 170%; color: #BBBdc8; font-weight: 100;
• Tooltips take ages to appear, it would be cool to reduce this time and make them almost directly to help users learning menus & tabs
• Missing padding (1.3em) in the Visual Editor
  nuxeo-view-editor.nuxeo-view-designer => background: #fff; width: 100%; padding: 1.3em;
• Would be cool to remove "Element Catalog” header bar (not necessary, we win space) but it will break accordion behaviour => Reduce accordeon header padding to .5em 2em; in Catalog tab and 0 1em; in Properties tab
• In Element Catalog list, you can add a border-bottom: 1px solid #BBBdc8; to catalog items to improve readability.