Allow multiple preconditions per cheat

Each cheat code can have multiple preconditions which all have to be fulfilled
before it can be executed. The default implementation in mupen64plus could not
handle these extra test codes because a failed test only caused a jump over the
next cheatcode (ignoring whether is also a test or a non-test cheatcode).

Therefore, all (negative) results before a non-test cheat code have to be
aggregated. The first non-test cheatcode evaluates the result of the
preconditions and only executes the non-test cheatcode when no precondition
test failed. The earlier results are always dropped after a non-test cheatcode
to allow a fresh start.
Use same precondition for all parts of the Mario64 Leviate cheat

The cheat code consists of three pairs of precondition and actual write. The
original code used two different preconditions (one for the first write and two
for the second/third write). This caused problems like the inability to finish
a level. Using the same precondition everywhere seems to fix this problem.
Merge pull request #10 from ecsv/cheats

Levitate cheat fixes