Introduce a proper Theme Settings API

I tried to follow this how-to to configure the settings for sub-theme but it doesn't work.

Besides, I tried both LESS and CDN ways to create sut-themes, unfortunately neither of these works as the front-page looks like a plain html even though I have copied all the settings from the base Bootstrap theme.

Any help?

[@YuanZheCSYZ] I've noticed that the JSDelivr setting under Admin > Appearances > [Bootstrap subtheme] > Advanced > CDN shows a version for 3.3.6 -- which is odd because there isn't one and the JSDelivr API isn't showing that. Be sure to choose 3.3.5 or earlier, then clear your cache.

Technically speaking (from core's point of view, see the related issue) this issue isn't a bug, it's a side-effect of the core "bug" that got "fixed"... yay :(

Basically, each sub-theme must also include a config/install/THEMENAME.settings.yml file. According to that issue, the "solution" is to simply copy the base-theme's settings file (yeah, real intuitive huh).

Personally, I think they have royally borked this entire process. It makes absolutely no sense that a sub-theme doesn't "inherit" it's base-theme settings just because they choose not to include this file.

I've discussed this with @neardark and have decided to completely refactor how this base theme handles "theme settings" in 8.x. Essentially we'll be ignoring theme_get_setting() (which no longer iterates of base theme settings) and create our own API.

This will kind of go hand in hand with this effort.

Not finished yet. Still have to migrate the CDN settings into the new @BootstrapSetting annotation discovery and also create base-theme inheritance for Theme::getSetting(). Setting to CNW.

Done:

• Moved cdn_provider and cdn_custom_* settings into @BootstrapSetting annotations.

Todo:

• Convert cdn_jsdelivr_* settings into @BootstrapSetting annotation.
• Replace Theme::getSetting() to utilize new @BootstrapSetting annotations.

This is now done :D

As stated above, due to how theme_get_setting() works, I definitely wouldn't recommend using it.

There is a before/after example on the bootstrap_setting() documentation page that can show you how to retrieve settings from a bootstrap theme in 8.x (given all the OO refactoring).

While this is mostly done, there still needs to be work done on detecting when not to use config (e.g. don't override inherited values that are the same).