Asserts for KWin scripts

Further debugging functionality for KWin scripts. Added assert
methods validate the to be tested parameter and throw a script
error if the value is not valid.

Following methods are available:
* assert(value)
* assertTrue(boolean)
* assertFalse(boolean)
* assertEquals(expected, actual)
* assertNull(nullValue)
* assertNotNull(notNullValue)

All methods take an additional optional parameter which is used
as the error message if provided.

Methods to validate the number of arguments and types of the
parameters are added and throw syntax or type errors.

REVIEW: 104870