Bugfixes/set timeout bug

I found, that under a certain condition a callback set by player.setTimeout is 
called twice. The bug can be reproduced by the following steps:

1. Add a long running timeout with setTimeout
2. Call a function from an ON_FRAME handler, which:
    
    a. Removes the long running timeout
    b. Adds a new timeout. The callback set with this timeout will be called twice.

```python
#!/usr/bin/env python

from __future__ import print_function
from libavg import player
from libavg import app


class MainDiv(app.MainDiv):
    def onInit(self):
        self.counter = 0
        self.calledOnce = False

        # long running dummy timeout. Will be removed before the timer elapses.
        self.longTimeoutId = player.setTimeout(10000, lambda: print('test timeout'))
    
        def onFrame():
            if not self.calledOnce:
                self.registerTimeout()
                self.calledOnce = True
        
        player.subscribe(player.ON_FRAME, onFrame)

    def registerTimeout(self):
        def onTimeOut():
            self.counter += 1
            print('onTimeout called {} times'.format(self.counter))

        # Remove the dummy timeout. 
        player.clearInterval(self.longTimeoutId)
        self.longTimeoutId = None

        # Now add a new timeout. This timeout will be called twice
        callbackId = player.setTimeout(100, onTimeOut)
        print('added timeout with id: {}'.format(callbackId))


if __name__ == '__main__':
    app.App().run(MainDiv())
```

The reason for this is, that the ON_FRAME signal is emitted from Player::handleTimers while a flag m_bInHandleTimers is set. This flag tells Player::clearInterval that it has been called from handleTimers 
and special care needs to be taken not to invalidate handleTimers iterators in case it is called by an currently processed time-out handler (notified by m_bCurrentTimeoutDeleted). 

Because the ON_FRAME signal is emitted after all timers have been processed, invalidating iterators is not an issue. Despite that, removing an timer and adding a new one, the latter will be flagged as already deleted and called twice on successive handleTimers calls.

To remedy this problem, m_bCurrentTimeoutDeleted needs be reseted before and not after each timer execution, as it is only of interest for the currently processed timer.