Avoid locale switching

[KjellMorgenstern]

Problem

ngspice uses the systems locale to read numbers.
However, we always use decimal dots when serializing numbers.

To avoid problems with the locale, we set it (and reset it) for each single call to ngspice.
This doesn't look right, although it seems to work.

Proposed Solution

Use locale LC_NUMERIC "C" for the complete Fritzing process.
We already introduced QLocale for the UI. There might be some places that don't yet apply this.
Also, be possible to set the QLocale in the preferences, e.g. when working with an international team.