Create a generic 'border_round' widget

[chshersh]

Create a widget that should do the following:

1. Takes Pretty.Layout.t
2. Wraps it around in a round border by adding a single space to the left and right sides

Put it in widget/generic.ml module.

The idea is to replace hardcoded widgets like this one:

github-tui/lib/tui/widget/issue.ml

Lines 10 to 18 in deb91b8

	let filter_open =
	fmt_filter
	~style:Style.issue_open
	~is_selected:(current_filter = Model.Issue.filter_open)
	[ "╭──────────╮";
	"│ Open o │";
	"╰──────────╯";
	] [@@ocamlformat "disable"]
	in

So let's also replace these three selector widgets once this new one is introduced.