Add New symbol action

[ra3xdh]

This PR contains the following improvements:

• Added New symbol action on toolbar as discussed in Improve SpiceLibComp device #679
• Implemented Schematic::isSymbolOnly property.
• Locked switching schematic/symbol if document is symbol only.
• Suggest the sym extension on Save As if document is symbol only. Not possible to save symbol as schematic. Saving schematic as symbol only works.
• Added new symnew.svg icon

[ra3xdh]

I have added the following fixes:

• The PortSymbol::numberStr must be always be a number. Otherwise it may cause symbol load issues.
• Allow delete port in symbol only mode