Move code to rename table to DBBrowserDB class

Move the code to rename a table from the EditTableForm to the
DBBrowserDB class where it should be and where it obviously was intended
to be.
Allow editing fields in existing DB tables

Make it possible to rename columns and change their types even when
editing an existing table. This requires a workaround to not use the
ALTER TABLE command which is not fully supported by SQLite.
Support dropping of columns

Support deleting columns of existing tables. Again this requires a
workaround because of SQLite's ALTER TABLE support.

Also show an error message to the user when editing a column failes.
Add keyboard shortcuts for the buttons in the browse tab again

Re-add the keyboard shortcuts for the find and refresh button in the
browse tab. They were lost in the process of using Qt Designer again.