Fix clear-screen timing when showing context (#1026)

According to doc, when enabled, before showing the debugging context,
screen will be cleaned.

```
Clear the screen before showing the context sections when breaking:
gef➤ gef config context.clear_screen 1
```

This is helpful to keep the debug experience focused, but the timing
seems to be broken with the current code.

Currently, clearing screen will happen after all contexts are shown.
Hence, whenever a breakpoint is hit, all contexts will be off the screen
leaving only the input prompt as screenshot shows below.

gef.py

@@ -7379,6 +7379,9 @@ def do_invoke(self, argv: List[str]) -> None:
         if redirect and os.access(redirect, os.W_OK):
             enable_redirect_output(to_file=redirect)
 
+        if self["clear_screen"] and len(argv) == 0:
+            clear_screen(redirect)
+
         for section in current_layout:
             if section[0] == "-":
                 continue
@@ -7400,9 +7403,6 @@ def do_invoke(self, argv: List[str]) -> None:
 
         self.context_title("")
 
-        if self["clear_screen"] and len(argv) == 0:
-            clear_screen(redirect)
-
         if redirect and os.access(redirect, os.W_OK):
             disable_redirect_output()
         return