Improve struct and pointer autocompletion in C (#4231)

* Add explicit trigger characters for C (#4226)

* Stop completion before issuing subsequent requests (#4226)

Co-authored-by: Marios Sioutis <26476573+s-marios@users.noreply.github.com>

test/python/test_deoplete_source.py

@@ -53,6 +53,7 @@ class DeopleteSourceTest(unittest.TestCase):
                 'rust': r'(\.|::)\w*$',
                 'typescript': r'(\.|\'|")\w*$',
                 'cpp': r'(\.|::|->)\w*$',
+                'c': r'(\.|->)\w*$',
             },
             'is_bytepos': True,
             'is_volatile': True,