Preferences window cleanups

[bragefuglseth]

For reference, this is how the preferences window currently looks 🙂

Some suggestions:

• Make it transient for the main window so it's attached to it
• Make the speed limit row an AdwSpinRow and settle on a single sensible unit instead of asking people to make the decision
• Make the username and password rows AdwEntryRows, and use the built-in confirmation button of the entry row instead of having one at the bottom
• Disable the search button because of the low amount of preferences

[giantpinkrobots]

Hello, I have followed through with three of your suggestions, and I think it does look better. But with the second suggestion, I hard disagree. I think being able to select between units is much better. Okay, maybe being able to select GB/s is a bit useless, but I think the KB/s and MB/s options can be very useful. Therefore I definitely want to have them.

Thank you for your suggestions! I am closing this now as completed. (I will submit these changes with the next commit)