Add options to number moves from variation or hotspot

[kevinsung]

Fixes #758 .

Adds options to number moves starting from

• The beginning of the current variation. The current variation starts from the most recent game tree node with more than one child. However, if the current position is in the main line, then move numbers are not shown.
• The most recent hotspot.

The screenshots below demonstrate the different options:

1. Number from game start.
2. Number from variation.
3. Number from hotspot