Swap UI: Fixed input bug due to locale differences

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Inputs allow locale dependent entry
Negative inputs are denied

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There was a problem with the inputs being impossible on different locales that does not use "." as the decimal separator. After some investigation, I realized the issue stemmed from trying to parseFloat the string input as the values entered were first converted into a string according to the local, but they could not be converted back to a number for locales that use ",". However, some work needs to be done before merging this, as parsefloat has precision problems. The conversion to string hid this, but internally, the precision is already lost when using float numbers.

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In this second commit for the pull request, I have fixed some useability problems in the Swap UI:

• You can now type "0" after the decimal separator. (e.g. you were not able to type 5.01, as 5.0 was immediately parsed back as 5.
• The input is completely decoupled from internal amounts, and this allows user to keep on inputting without being affected from changes to the fair price during input
• The amounts internally use big number types, so floating point rounding errors do not produce unintended input when inputting big numbers or many numbers after the decimal separator, would allow more robust and feature proof input.