Redesign simulator switching system

[zergud]

fixes #274

[ra3xdh]

I have tested this PR and found the following problems:

• New system cannot recognize simulator if it is defined using $PATH. This case is common for Linux platform. For example it cannot find qucsator on my Linux system.
  

I suggest to implement a custom search procedure instead of using simple QFile::exists(). It should look for executable also in $PATH.

• An addition to the previous point. I have implemented an automatic search of the Ngspice exexcutable in order to prepare application for the build of Windows installer. This feature may be combined to simulator selection. See 7a77ceb

• It is possible to set simulator to Not specified and simulation doesn't work in such case. Only simulators should be available in the drop-down list.
  

• On the first start the applications reads old settings from config file and sets simulator to Not specified. As the solution I may suggest to rename the setting in the config (see saveSettings() and loadSettings in main.cpp). Also it may help to shift bit mask of the spicecompat::Simulator The range from 0x00 to 0x0F will indicate old settings before v1.1.1 and it's need to show the settings dialog on the first start again.

• The blue BJT, MOSFET, and JFET symbols should be available for all simulators, but not only for Qucsator.

[ra3xdh]

Now everything seems to work as expected except the problem with simulators in $PATH I will try to provide the solution for this problem in the next few days.

[ra3xdh]

See https://github.com/zergud/qucs_s/pull/2 for solution of the $PATH search problem.

[ra3xdh]

I have tested it again and found no remaining problems. Merging this. Thanks for the contribution!