Various fixes for mingw-builds

* Make Set-Compiler accept "mingw-builds" argument
* add gnuwin32\bin to path (for win_flex)
* Don't use SETQT_PATH for Get-QtBasePath. Its not really necessary
  since the fallback should be sufficient
* move the default setqt to shell.ps1
* Make sure the shell.ps1 actually uses Set-Compiler and not the legacy
  setcompiler