Add simple border

[gladmo]

Widget Grid can use SetSimpleBorder() to set the simple border.
look like:

+----------+--------------+------+-----+---------+----------------+
|  FIELD   |     TYPE     | NULL | KEY | DEFAULT |     EXTRA      |
+----------+--------------+------+-----+---------+----------------+
| user_id  | smallint(5)  | NO   | PRI | NULL    | auto_increment |
+----------+--------------+------+-----+---------+----------------+
| username | varchar(10)  | NO   |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+
| password | varchar(100) | NO   |     | NULL    |                |
+----------+--------------+------+-----+---------+----------------+

default

┌────────────┬──────────────┬───────┬───────┬──────────┬──────────────────┐
│FIELD       │ TYPE         │ NULL  │ KEY   │ DEFAULT  │ EXTRA            │
├────────────┼──────────────┼───────┼───────┼──────────┼──────────────────┤
│ user_id    │ smallint(5)  │ NO    │ PRI   │ NULL     │auto_increment    │
├────────────┼──────────────┼───────┼───────┼──────────┼──────────────────┤
│ username   │ varchar(10)  │ NO    │       │ NULL     │ dog              │
├────────────┼──────────────┼───────┼───────┼──────────┼──────────────────┤
│ password   │ varchar(100) │ NO    │       │ NULL     │ dog              │
└────────────┴──────────────┴───────┴───────┴──────────┴──────────────────┘

[marcusolsson]

I think this is a nice addition! Although ideally I think this could be considered a global theme setting (I don't see people using both default and simple border in the same application, but I could be wrong. I'm thinking possibly adding a method to the Theme struct (maybe tui.SetBorderStyle(tui.BorderStyleSimple)). Then, instead of checking the border style in each widget, maybe add a new tui.DrawRectangle() to the Painter struct. Since Painter has access to the current theme, you could do the lookup there instead. What do you think?

[gladmo]

Thanks for your advice.

Before submitting this pr, I thought there were many ways to implement this feature without breaking the original design. One scene that makes me want to use two borders at the same time is that I use tui-go to create a terminal app which has a login page and many forms. The login in example is very pretty, you can display different forms after login. The report uses a simple border to make it look easier.

Making changes in the Painter allows for more flexible functionality and more considerations. Now that I have some good ideas for adding functionality to Painter.