fix(core): make the % works correctly

[iovxw]

Just like KCalc and GNOME Calculator.

Closes #12

[iovxw]

5000 - 30% 等价于 5000 - (5000 * 0.3)

仅限于 + -，和其他主流计算器行为保持一致

BTW: 由于使用的是 SpeedCrunch 开启了 ALLOW_IMPLICIT_MULT， 所以 100%100 是合法的

[BLumia]

cc @hualet

[z966768]

2个问题：1.我们当前版本只支持标准模式，标准模式的处理由于之前的需求，直接出结果，逻辑一般的计算器不同，例如5000-30%，直接会显示为5000-0.3，不会合起来计算为5000-(5000×0.3)。如果要修改，可能得提新的需求。2.这一笔代码合入后，计算5000-（30%）的值与5000-30%的值相同，和一些计算器有出入，需要考虑，故当前版本不予合入

[hundundadi]

After our internal analysis, this modification will cause the existing calculation logic of the calculator to change and does not conform to the calculation logic, so this PR cannot be incorporated.

[deepin-bot]

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