Refactor dialect subpackage: rename fields and methods for clarity

[hymkor]

• Rename dialect.Entry.Columns to FetchColumns
• Rename dialect.Entry.Tables to FetchTables
• Rename dialect.SqlToQueryTables to BuildSQLForTables
• Rename dialect.SqlToQueryColumns to BuildSQLForColumns
• Add comments to the field and the method of dialect.Entry
• Rename dialect.Entry.CanUseInTransaction to IsTransactionSafe
• Rename dialect.Entry.ColumnField to ColumnNameField
• Rename dialect.Entry.TableField to TableNameField
• Rename dialect.Entry.SqlForTab to SQLForTables
• Rename dialect.Entry.SqlForDesc to SQLForColumns`
• Rename dialect.Entry.TypeToConv to LookupConverter
• Rename dialect.Entry.TypeNameToConv to TypeConverterFor
• Replace the function dialect.Register to the method dialect.(*Entry) Register
• Removed unused field: dialect.Entry.DisplayDateTimeLayout