Narrow String properties to a more precise type if possible

[snicoll]

The IDE support has now smart support for Charset, MimeType, Resource and Locale. Looking at the code now, most of them are exposed as String and the IDE support is tracking them "manually" to offer auto-completion.

It would be much nicer if we moved those keys to their relevant types so that the IDE can just pick up that in a generic fashion.

[snicoll]

Current candidates:

banner.location=classpath:banner.txt # (java.lang.String) - Banner file location.
spring.config.location= # (java.lang.String) - Config file locations.
server.tomcat.compressable-mime-types=text/html,text/xml,text/plain # (java.lang.String) - Comma-separated list of MIME types for which compression is used.
server.tomcat.uri-encoding= # (java.lang.String) - Character encoding to use to decode the URI.
liquibase.change-log=classpath:/db/changelog/db.changelog-master.yaml # (java.lang.String) - Change log configuration path.
spring.datasource.sql-script-encoding= # (java.lang.String) - SQL scripts encoding.
spring.freemarker.charset= # (java.lang.String) - Template encoding.
spring.freemarker.content-type= # (java.lang.String) - Content-Type value.
spring.groovy.template.charset= # (java.lang.String) - Template encoding.
spring.groovy.template.content-type= # (java.lang.String) - Content-Type value.
spring.mail.default-encoding=UTF-8 # (java.lang.String) - Default MimeMessage encoding.
spring.messages.encoding=utf-8 # (java.lang.String) - Message bundles encoding.
spring.mustache.charset= # (java.lang.String) - Template encoding.
spring.mustache.content-type= # (java.lang.String) - Content-Type value.
spring.mvc.locale= # (java.lang.String) - Locale to use.
spring.thymeleaf.content-type=text/html # (java.lang.String) - Content-Type value.
spring.thymeleaf.encoding=UTF-8 # (java.lang.String) - Template encoding.
spring.velocity.charset= # (java.lang.String) - Template encoding.
spring.velocity.content-type= # (java.lang.String) - Content-Type value.

[snicoll]

Annoying side effect, we lose the detection of default value with this. Maybe we could improve it...