Change design of DBBrowserDB a bit and support more actions with views

Change the design of DBBrowserDB to store all DB objects in one single
map instead of having one map for table, one for views, one for triggers
etc. This requires some changes in the entire program.

Show more details of views in the DB structure tab, i.e. the names of
the fields included in the view. Allow browsing views in the browse tab
and allow exporting views as CSV.
Allow deletion of indices, views and triggers

Extend the code used to delete tables to also allow the deletion of
other objects in the data base.

Change the activation of the popup menu entries to activate the
associated action when a trigger, view or index is selected.
Delete the DeleteIndexForm

Delete the DeleteIndexForm as it is no longer needed. Since the last
commit an index can be deleted directly via the popup menu.