Xlet settings dependencies with operators

This is something I wanted very early for my applet settings: dependencies with some operators.
Those dependence values are now possible:

- `!` and checkbox key: enabled if checkbox is not checked
- only combobox or radiogroup key: enabled if value can be cast to true
- `!` and combobox or radiogroup key: enabled if value can be cast to false
- combobox or radiogroup key, `==`, `!=`, `<`, `>`, `<=` or `>=` and a value: enabled if statement is true

##### Examples
`!use-foo`
`dropdown`
`bar-mode < 5`
`divisor != 0`