Optimization: Remove Lib.getTimer() as much as possible in code

Lib.getTimer() is a very expensive call. Unless we're doing performance profiling it should be called once in the code and cached.

In FlxGame.hx stores this value already in onEnterFrame:
```
_marks = Lib.getTimer()
```

I suggest we make FlxU.getTicks() refer to this value, and replace all calls to Lib.getTimer() with FlxU.getTicks() throughout the code.

Summary of changes:
* Removed all instances of Lib.getTimer() outside of FlxGame.hx, FlxPreloader.hx and Perf.hx.
* Made FlxWeapon's rate of fire functionality work with custom timeScales.