Fix issue 418: Clicking Move after Remove casuse VS Crash
Fix issue 418: Clicking Move after Remove casuse VS Crash

Move Up/Down buttons were not enabled/disabled appropriately. This caused VS to crash if movement would cause invalid index.

- Call UpdateUI() after OnAddButtonClick() and OnRemoveButtonClick() to set correct Enabled state
- Add validation in MoveSelectedItem() to ensure index in bounds
- Remove non-focus gradient color since it looked odd after clicking move buttons (blue to gray)

--HG--
branch : 1.0
extra : rebase_source : 94ee2d7653bfa37cb57cbe9d9aa350c5d6e34058

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