Keeping QgsTask classes in one file consistently

[Gpetrak]

Currently the QgsTask classes (e.g LayerLoaderTask, ThumbnailLoaderTask or NetworkRequestTask) are included in different files (e.g network.py, search_result_widget.py). However, for consistency reasons, we could keep all the QgsTask classes in one file e.g tasks.py.

Also the LayerUploaderTask task could be refactored in order to not include repetitive code which comes from its base class: NetworkRequestTask.

[Gpetrak]

@giohappy the issue is completed and you can find the current PR here: #304
The changes that are included are the following:

• I kept all the tasks in a new folder, called: tasks
• In this folder I kept the main task (network_task) in a separate file
• I included all the other tasks in the tasks/tasks.py
• I kept / moved all the functions related with the network in the network.py
• All the functions related with the style are in the styles.py
• I moved all the other functions in the utils.py