During #18762, complained about not knowing what to enter in the text field for the bridge line (from a build of !905). They wondered whether it was a line to enter their email address. - [x] Add a label: **Bridge** on the left of the text input - [x] Add a `obfs4 ...` placeholder inside the input, if that's easy using [GtkEntry:placeholder-text](https://docs.gtk.org/gtk3/property.Entry.placeholder-text.html)