Refactor Property class

[ra3xdh]

The Property (see element.h) class is inherited mostly unchanged from Qt3 application version and may benefit from the refactoring. I propose the following things:

• Add a field containing property type: file, equation or usual property
• Add a field containing allowed simulator type: SPICE, Qucsator, etc.

The implementation of the second will allow to hide simulator-incompatible device properties from user to prevent its accidental setting and usage.