Implement a hack to (mostly) preserve tabs when cat a file with them and then copying the text or passing screen contents to another program

It's a simple enough hack that it seems worth doing. If it causes any
issues, can always be reverted.

Fixes #1829

kitty_tests/screen.py

@@ -293,11 +293,7 @@ class TestScreen(BaseTest):
             s.tab()
             s.draw('*')
         s.cursor_position(2, 2)
-        for col in range(2, s.columns - 1, 6):
-            for i in range(5):
-                s.draw(' ')
-            s.draw('*')
-        self.ae(str(s.line(0)), str(s.line(1)))
+        self.ae(str(s.line(0)), '\t*'*13)
 
     def test_margins(self):
         # Taken from vttest/main.c
@@ -338,7 +334,8 @@ class TestScreen(BaseTest):
                     s.cursor_position(region, s.columns), s.draw(ch.lower())
             for l in range(2, region + 2):
                 c = chr(ord('I') + l - 2)
-                self.ae(c + ' ' * (s.columns - 2) + c.lower(), str(s.line(l)))
+                before = '\t' if l % 4 == 0 else ' '
+                self.ae(c + ' ' * (s.columns - 3) + before + c.lower(), str(s.line(l)))
             s.reset_mode(DECOM)
         # Test that moving cursor outside the margins works as expected
         s = self.create_screen(10, 10)