SqliteTableModel: Do a few fixes and add sorting again

Fix empty rows being shown in the main window when changing to a table
with less rows than the old one.

Add sorting in the browse tab again.

Do some minor clean ups.

src/MainWindow.ui

@@ -312,7 +312,7 @@
          <rect>
           <x>0</x>
           <y>0</y>
-          <width>763</width>
+          <width>278</width>
           <height>444</height>
          </rect>
         </property>