Add subscription: Design feedback

[TriVoxel]

I've noticed a couple minor problems with adding subscriptions that could use some polish.

• When the menu pops up, the GtkTextEntry should be immediately selected so the user doesn't need to tap or click.
• The last provider selected should be remembered. This would be useful, ie. when someone is adding LBRY creators, they don't need to keep selecting LBRY every time
• Pressing Enter from within the GtkTextEntry should "Add" the entry