[NFR]: Align bar icon names and drawings #64

The icon knowledge is split between PHP and JavaScript, and nothing keeps the two sides in line.

• Collectors set an icon name in PHP. debugbar.js holds the drawings and hard-codes search.
• indicatorIcon() checks only for clock and search. Every other name gets the gears, so a typo in a collector draws gears with no error.
• indicator.icon is a plain string, so static analysis accepts any value.

Changes:

• Replace the if chain in indicatorIcon() with one map from name to drawing.
• Add an icon_type list of the valid names to DebugBarTypes, and use it for indicator.icon.
• Add a test that fails when the PHP names and the JavaScript map do not match.
• Show no icon for a name that the map does not have, so that a custom collector with an unknown name does not get the gears.

Tabs stay as text, because icons would make each button wider.