Enhance the UI

[nicomazz]

The graphic is pretty much terrible.

• [ ] The top bar layout is pretty bad, it should be fixed and made better
• [ ] There are problems with the layout in small screens
• [ ] The contrast of "Hexdump" is wrong (it is black now, but it should be white
• [ ] The popup for copying a request to pwntools or python-requests looks ugly, it should be nicer
• [ ] When there aren't flows, there should be a placeholder with the instruction to import them (light gray, as the one in empty lists)
• [ ] When there are flows, but none is selected, there should be a placeholder with written "Select a flow from the left menu"
• [ ] Add flag icon in the flows with a flag