Fix validation error for the decimal quantity field

[lanresmith]

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Fix validation error for the decimal quantity field when the value starts with the decimal sign (dot/comma).

As can be seen in the screenshot below, form validation fails for a value like .2 - which is a valid value - even when the total is correctly calculated. This is quite inconsistent.

Code formatting/refactoring was also done for some parts of the existing code in order to provide clarity and reduce repetitions e.g. the GFCommon::is_numeric method was used 4 times in the old code but just twice in the new code.

Context

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Summary