make "reset" button insensitive if nothing to reset

in the adjust dialog, we should make the "reset" button insensitive if the photo has already been reset and this button will have no effect.

This is important to give the user confidence that photo has actually be reset to its original adjustments.

Launchpad Details: [#LP1360068](https://bugs.launchpad.net/bugs/1360068) Daniel Fore - 2014-08-22 05:05:44 +0000

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The issue names the adjust dialog and reset button; start by locating that dialog's reset action and the state that indicates original adjustments. Done means the button is insensitive when no reset would change the photo, and becomes usable when adjustments can be reset; verify both states in the UI.

Written by the indexing model from the issue text.