Add 2's complement negative support

[MrHate]

#10

This commit might not be well enough considered or complete.

• I added a function decimal2negative to convert a decimal larger than 1 into a negative decimal under the 2's complement principle with the zeros before the most significant bit trimmed.
• I displayed the result within the forth row when the given argument evaluated larger than decimal 1 for all convert scripts except for the "number convert".
• I reused the decimal icon.
• Also not well tested for extreme values.

[obstschale]

Just a quick note. I did not forget about your PR, but work is currently keeping me busy. Thanks for your patience

[MrHate]

It's ok, thanks for this quick note :)