Explicitly set the QLocale in the GUI

[MBartkowiakSTFC]

Description of work
The number format will always follow the British convention, where "1 point 0" is 1.0 and not 1,0 or anything else.

closes #595

Fixes

1. Created a QLocale instance set to English, UK, before starting the GUI.

To test
On a platform already set to English, there should be no difference.
On platforms using different symbols for the decimal point, the MDANSE GUI should now accept the '.' character instead.

[ChiCheng45]

I've tested this on a German Windows 11 VM and this change allows me to use '.' characters as the decimal point.

One problem I can see is if they use ',' as the decimal point then QT converts it to a different number e.g.

1,3 -> 1.3E+01

This might be confusing to new users. What do you think about disabling the use of commas altogether?

[MBartkowiakSTFC]

Unfortunately, commas are allowed in English number formatting, as in "1,000,000.05" so a normal QDoubleValidator will not remove them. But you are right, they should be disabled.

[MBartkowiakSTFC]

As far as I can tell, this should work now.

[ChiCheng45]

Looks good.