Adds reverse tabIndex direction when using shift-tab

[luke-ransfield]

Just for fun, I saw it and thought I'd it implement it.

Changes:

• uses OnKeyDown instead of OnKeyPress so can use e.KeyCode == Keys.Tab and e.Shift (can revert to using e.KeyChar if that's preferred).
• Used NextControl as base for PreviousControl
• PreviousControl returns previous control by iterating through controls, storing reference to control in previousKey, until it breaks at current control
• Handles if If current control is first in dictionary, will iterate through entire dict to get the last control.

To test:

• run SampleCsModelessTabForm command, check that tab press increments tab index, check that tab-shift reverses tab index direction

Tried to keep consistent code style with existing. Also not sure what your contributions policy is (no worries if you don't accept PRs!)