Rework alert (danger/attentions/HV) symbols

Rework alert symbols (attention, danger, high voltage):

After:

Includes:

• Unify sizes between symbols
• Use ISO-style graphics (including using equilateral triangles)
• Tidy up descriptions and keywords
• Hide ref des and values
• Some HV symbols renamed ( Highvoltage to HighVoltage - capital V)

Some graphics are from https://en.wikipedia.org/wiki/ISO_7010, from images declared to be public domain.

Expected KLC failures

• F5.1: SIlkscreen line width - some graphics are drawn as filled polygons with no line width
• F5.2: Fab value not included - hidden fab value text (some were already like this)
• F5.3: No courtyard for a graphic
• F9.3: NO 3D model for a graphic