* Implement base background image alignment settings
TerminalSettings now has two new properties:
* BackgroundImageHorizontalAlignment
* BackgroundImageVerticalAlignment
These properties are used in TermControl::_InitializeBackgroundBrush to specify the alignment for TermControl::_bgImageLayer.
This is a base commit that will split into two possible branches:
* Use one setting in profiles.json: "backgroundImageAlignment"
* Use two settings in profiles.json: "backgroundImageHorizontal/VerticalAlignment"
* Implement background image alignment profile setting
Implement background image alignment as one profile setting.
* This has the benefit of acting as a single setting when the user would likely want to change both horizontal and vertical alignment.
* HorizontalAlignment and VerticalAlignment are still stored as a tuple in Profile because they are an optional field. And thus, it would not make sense for one of the alignments to be left unused while the other is not.
* Cons are that the tuple signature is quite long, but it is only used in a small number of locations. The Serialize method is also a little mishapen with the nested switch statements. Empty lines have been added between base-level cases to improve readability.
* Fix capitalization typo for BackgroundImageStretchModeKey
In Profiles.cpp, the key for the image stretch mode json property had a lowercase 'i' in "Backgroundimage", not following proper UpperCamelCase.
The "i" has been capitalized and the two usages of the constant have been updated as well.
* Document Background Image settings
* Adds entries SettingsSchema.md for the original 3 backgroundImage settings in addition to the new backgroundImageAlignment setting.
* Fix setting capitalization error in UsingJsonSettings.md
* The background image example in UsingJsonSettings.md listing a backgroundImageStretchMode of "Fill" has been corrected to "fill".
Fixes#1949.
