Add lens control to unit action bar

**Issue by [jeroextran](https://github.com/jeroextran)**
_Sunday Apr 16, 2017 at 17:02 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/503_

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This is an enhancement request.

**Issue description:** Unit lens view is by and large a modal action that is arguably unnecessary much of the time. But if lenses are defaulted to off for, say, builders, going into the lens view is too much clicking and scrolling for what should be a quick mode change.

**Request:** Add a button to the relevant units' action bar to toggle lens view on and off. This would permit the lens to stay off by default, allowing easy switching to lens mode when needed to choose the best plot.

As part of this request, all units with lens views should have both unit bar buttons and Options menu settings to toggle the default state. Thus settlers and religious units would need toggles added in the Options menu.

Locate the unit action bar, existing lens-view handling, and the Options menu settings for settlers and religious units; start by tracing how current lens actions and default states are defined. Done means relevant units have an action-bar toggle and all units with lens views have an Options setting for their default state.

Written by the indexing model from the issue text.