Fix half a dozen bugs concerning the find dialog

Fix bug that caused invalid rows in the result table view. This could
happen because the rowid of the search result was used as the row number
in the table widget, too. Every rowid, however, is basically a random
number and to address the rows in the table widget you need continuos
numbers.

Fix bug that made it impossible to click on any column but the first one
in the result table widget. Clicking on a column in the data column lead
to wrong rows being highlighted because exactly that text was used that
the user clicked on instead of taking the text of the first column of
the row the user clicked on.

Make it impossible for the user to edit the cells of the search result
table widget. Editing these not just only makes no sense at all but can
also lead to wrong rows being selected in the main window when the rowid
in the first column was changed.

Fix bug that caused the wrong row to be selected in the main window.
When the table view in the find dialog is populated the rowid shown is
increased by one for every search result. This is fair enough but then
this modified row number needs to be changed back before selecting it.
Not doing so leads to always selecting row right after the intendet one.

Fix bug when searching for entries containing a number in some field.
This was caused because the search string was changed to be surrounded
by %-signs as you'd expect when doing a containing-search. However, the
check wether we're searching for a number or not was done on the old
string. So checking for a number was positive even though the program
would actually search for a string. This way the string was not put into
'...'-signs, leading to a malformed SQL statement.

Clear all input widgets as well as the result table widget when
(re)opening the find dialog. This may be less comfortable in some
situations but at least is a bit more consequent because the column
combo box was reset anyway.