Timeseries widget: Test if layerChanged is connected before trying to disconnect

[JoerivanEngelen]

Fixes #54 , there is a bug where for vector data the plugin attempted to disconnect the layerChanged signal, but threw an error because nothing was connect to this signal.

To reproduce this issue:

• Select vector data in timeseries widget
• Hide QGIS
• Unhide QGIS
• Hide QGIS

In my experience, the signalspy is the easiest way to track whether things are connected to a signal or not.

[JoerivanEngelen]

UPDATE:

Signalspy only records signals after being initiated, so my first implementation in this branch doesn't work (it expected that all signals connected were traced back, but this is wrong). I also found that it is possible to connect the same method 5 times to a signal, after which the disconnect method also has to be called 5 times to fully disconnect.

So I resorted to looping over the amount of receivers and try / excepting disconnecting the method. I also made sure the method is reconnected again during the showEvent.