Rename FrameSequencer to FrameGrabber and break out into separate file.

Add a test case that the bmp2dhr output of input filenames containing
'.'  are handled correctly.

Break out video.Mode into video_mode.VideoMode to resolve circular
dependency.

transcoder/video_mode.py

@@ -0,0 +1,8 @@
+"""Enum representing video encoding mode."""
+
+import enum
+
+
+class VideoMode(enum.Enum):
+    HGR = 0
+    DHGR = 1