More fixes, lesser "caching" and more wrapper functions introduced:

More fixes, lesser "caching" and more wrapper functions introduced:
- Wrapper function addExtensionAddTableIndexSql() introduced
- Wrapper function addExtensionAddTableUniqueSql() introduced
- Wrapper function addExtensionAddTableFulltextSql() introduced
- Wrapper function addExtensionDropTableIndexSql() introduced
- Function ifSqlTableExists() renamed to ifSqlTableColumnExists() and it does
  no longer "cache" its results
- Other fixes which prevents extensions being installed/uninstalled