Create Library from Spice library device

[ra3xdh]

This PR resolves a restriction reported by #944 The automatic extraction of SPICE .SUBCKT code from SPICE libraries is not possible, because some subcircuits may have complex dependencies. It is not possible to process these dependencies without implementing a full SPICE parser. So, I propose the following logic:

• The SPICE File device works as before. The user must extract the subcircuit code with dependencies manually and put it in a separate file for every subcircuit. One device represent one subcircuit. The SPICE code is embedded in Qucs XML library in <Spice> </Spice section when creating the library contaning the subcircuits with SpiceFile devices.
• The SPICE Library device is only referencing the external SPICE library instead. When creating library, the application checks if the schematic contains Spice Library device and copies the library file to the subdirectory with the same name as the library name. The SPICE libraries relative file names are referenced in <SpiceAttach> section of the XML file. The attached library and source project illustrate this workflow. Copy the library files to the $HOME/QucsWorkspace/user_lib. The SPICE library is in TriacST/ subdirectory.

Thyristor_ST_prj.zip
TriacST.zip

Besides the Spice Library Device extension, this patch adds an Analog models only switch to the Create library dialog. This is active by default and suppress the generation of dummy digital VHDL/Verilog models for analog devices.