Fixed registry cleanup and adjusted pair list display/handling in Options dialog.

[dansyl1]

• Improved unused registry key cleanup (some keys were left behind).
• Sort of items in pair list now case-insensitive.
• Items in the pair list's context menu now enabled/disabled as needed (same as buttons).
• Added validation when creating pairs.
• Added validation when editing pairs.
• Internally, the index of items in the pair list is kept in the lParam.
• Internally, pair vector is no longer sorted, sorting is not required and doing so may change item's positions in the vector and make lParam values in displayed list invalid.
• Directory names comparisons for pairs now case-insensitive.