Downloader with GUI

Addition of a graphical interface for the downloader.
For now, the GUI supports only the following filtering parameters:

• tile identifiers (both S2 and L8)
• sensing start/end dates
• cloud percentage
• relative orbit

In a future release, there will be added:

• download by product name
• filtering using a WKT for the area of interest
• proxy parameters

Corrections:

• fix relative orbit padding for Amazon search