Rework breaking API changes

This commit changes the new mode-based configuration to use two new
methods – setBannerMode on SpringApplication and bannerMode on
SpringApplicationBuilder. The old methods, setShowBanner and
showBanner on SpringApplication and SpringApplicationBuilder
respectively, have been reinstated and deprecated.

Closes gh-4001

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -43,6 +43,7 @@ content into your application; rather pick only the properties that you need.
 	spring.main.sources= # sources (class name, package name or XML resource location) to include
 	spring.main.web-environment= # detect by default
 	spring.main.show-banner=true
+	spring.main.banner-mode=console # the mode used to display the banner (console, off, or log)
 	spring.main....= # see class for all properties
 
 	# AUTO-CONFIGURATION

spring-boot-docs/src/main/asciidoc/howto.adoc

@@ -123,7 +123,7 @@ might have.
 [source,properties,indent=0,subs="verbatim,quotes,attributes"]
 ----
 	spring.main.web_environment=false
-	spring.main.show_banner=OFF
+	spring.main.banner_mode=off
 ----
 
 and then the Spring Boot banner will not be printed on startup, and the application will
@@ -139,7 +139,7 @@ consider this application
 [source,java,indent=0]
 ----
 	new SpringApplicationBuilder()
-		.showBanner(Banner.Mode.OFF)
+		.bannerMode(Banner.Mode.OFF)
 		.sources(demo.MyApp.class)
 		.run(args);
 ----
@@ -149,7 +149,7 @@ used with the following configuration:
 [source,properties,indent=0,subs="verbatim,quotes,attributes"]
 ----
 	spring.main.sources=com.acme.Config,com.acme.ExtraConfig
-	spring.main.show-banner=CONSOLE
+	spring.main.banner_mode=console
 ----
 
 The actual application will _now_ show the banner (as overridden by configuration) and use

spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -93,7 +93,7 @@ instance and customize it. For example, to turn off the banner you would write:
 ----
 	public static void main(String[] args) {
 		SpringApplication app = new SpringApplication(MySpringConfiguration.class);
-		app.setShowBanner(false);
+		app.setBannerMode(Banner.Mode.OFF);
 		app.run(args);
 	}
 ----
@@ -124,7 +124,7 @@ For example:
 [source,java,indent=0]
 ----
 	new SpringApplicationBuilder()
-		.showBanner(Banner.Mode.OFF)
+		.bannerMode(Banner.Mode.OFF)
 		.sources(Parent.class)
 		.child(Application.class)
 		.run(args);