Several new features for schema comparison

Update the schema comparison by following changes

1) Avoid to open a new tab if the table schema comparison is already appeared, back to the existed tab and refresh the compare contents

2) Append a context menu to the table cells in order to copy alter DDL script (supports multiple selections)

3) Add a refresh button on the detailed comparison view, remove "Switch General View" button. The refresh button reloads the db schema comparison information if there is any change (e.g. after the alter DDL executed etc.)