A conditional check for non default BG colors will prevent alpha being applied to them. `_defaultBGAlphaOnly` will reference a user setting(checkbox) to make this feature opt-in.

Height is adjusted for the `77f-eM-Dma`/`Prefs - Profiles - Window` to make room for new checkbox element. Doing so adjusted all the `y` position values.

I added the checkbox with copy/paste of the previous element within the XCode UI editor and updated the properties. 

I could not find how to add the `outlet` hunk via that interface so directly added it by editing the XML. The id I used for the `outlet` node doesn't have any conflicts and is presumably safe.

Points the `_preventTab` node to `_defaultBGAlphaOnly` node. 

Also added a `nextKeyView` connection from `syncTitle` to `_preventTab` as it was missing one.

This finishes the feature off by connecting the UI checkbox setting to the first commit that introduced the `_defaultBGAlphaOnly` variable.

I've tried to keep the location of additions consistent, either being inserted after `blend` or `_preventDefault` logic.

As per request: gnachman#277 (comment)

As per request: gnachman#277 (comment)

to `setTransparencyAffectsOnlyDefaultBackgroundColor`.

As per request: gnachman#277 (comment)

As per request: gnachman#277 (comment)

Use American English spelling.

As per request: gnachman#277 (comment)

to `KEY_TRANSPARENCY_AFFECTS_ONLY_DEFAULT_BACKGROUND_COLOR`.

As per request: gnachman#277 (comment)