Initial terminal ui implementation

- This commit adds proof of concept work for terminal ui as is.
- Some things work, some don't but we need to start from somewhere.
  Further development continues in a main.
- Essentially we are starting to have enough so that it merits to
  move all this work into a main repo.
- Everything new is kept under org.springframework.shell.component.view
  and will get revisiter later to find correct locations for some classes.
- Catalog sample has been modified to provide "showcase" app for
  terminal ui features. This is a start while it already contains
  some usefull scenarios.
- Relates #800
- Relates #801
- Relates #802
- Relates #803
- Relates #804
- Relates #805
- Relates #806
- Relates #807
- Relates #808
- Relates #809
- Relates #810
- Relates #811

spring-shell-core/spring-shell-core.gradle

@@ -10,6 +10,7 @@ dependencies {
 	api('org.springframework:spring-core')
 	api('org.springframework.boot:spring-boot-starter-validation')
 	api('org.springframework:spring-messaging')
+	api('io.projectreactor:reactor-core')
 	api('org.jline:jline')
 	api('org.antlr:ST4')
 	api('commons-io:commons-io')
@@ -17,4 +18,5 @@ dependencies {
 	testImplementation 'org.springframework.boot:spring-boot-starter-test'
 	testImplementation 'org.awaitility:awaitility'
 	testImplementation 'com.google.jimfs:jimfs'
+	testImplementation 'io.projectreactor:reactor-test'
 }