Moved graphics classes into own namespace, added unit testing with gtest

tests/meson.build

@@ -0,0 +1,14 @@
+tests = [['bitmap_test', 'bitmap_test_exe', ['bitmap_test/bitmap_test.cpp']],
+         ['image_test', 'image_test_exe', ['image_test/image_test.cpp']]
+        ]
+
+foreach test : tests
+    test(test.get(0),
+         executable(test.get(1),
+                    sources: test.get(2),
+                    include_directories: incdir,
+                    link_with:[ptprnt_lib],
+                    dependencies: [gtest_dep, pangocairo_dep]
+         )
+    )
+endforeach