implement cache to remember last selected

[Lemorz56]

Implement a cache for the GUI mode to remember last interface and pcap file selected if we start the GUI again. Try to see if we can use fyne's storage or just use a simple file in the appdata dir.

[krishnaduttPanchagnula]

remember last interface

Can you expand what do you mean by last interface? Is it fine if we just remember the path of the previous pcap file?

[Lemorz56]

remember last interface

Can you expand what do you mean by last interface? Is it fine if we just remember the path of the previous pcap file?

By interface I mean the Network Interface, which right now is stored in the variable "commons.IntfId". Used for example in pcap.go on line 198