Implement an extended table widget to add proper copy to clipboard support

Extend the standard table widget class to make it possible to copy
multiple cells to the clipboard.
Make the QTableWidget a QTableView

Rewrite some code to use a QTableView widget instead of a QTableWidget
in the browse tab of the main window, or more presicely in the new
extended table widget class. This way the widget and the data model are
separated which on the one hand means a bit more work on our side but on
the other hand also gives us a bit more flexibility. And more
importantly at the moment: This means both, the browse tab and the sql
tab, use a table view now - that's going to be helpful for the next
commit...
Add proper copy support to the SQL tab table view widget

Allow copying data from the result table view widget in the SQL tab of
the main window.
Fix selection of entire columns in browse tab

Fix the selection of entire columns in the browse tab table view. Even
when selecting multiple columns the data would be re-sorted instead of
selected; this is changed now.
Merge pull request #7 from MKleusberg/improved-copy-paste

Improve the copy to clipboard feature quite a bit