Use TreeView to render the completions (#39).

This seems to work, but there are some known issues with gtk3 at the moment.
- The tree view widget size could not be set, so that tree items could be
  hidden if there are to many of them in completion
- The active styles are not applied if used with gtk3
- The tree view shrinks to on single item and becomes unusable on some urls of
  the history - seems there are some char in them that break the tree view or
  the column renderer

Not that the configuration for max-completion-items was removed that could not
be applied to show the tree view.
Removed direct reading of tree by select function.

This seems to be the better way to get the text of the current active
completion items instead of moving the cursor to the new item and that try to
get the text of it.
Removed completion tree view widget from global scope (#39).
Use fixed height mode for tree view that's faster (#39).
Put the completion tree view into scrolled window (#39).
Fixed non highlighted completion item with gtk3 (#39).
Renamed update into move_cursor.
If completion is started backwards mark the last item (#39).

If the completion where started in reverse order by <shift-tab> the first item
was marked as active instead of the expected last one.
Split tree model creation from preparing the widget (#39).
Fixed broken completion for long uri (#39).

If the completion reached a uri that where longer than 255 chars the
completion reduced to one possible item. The reason was the vb_echo* functions
that used only 255 chars to put into inputbox. If the uri was longer we
considered the current text and the text of completion item as different and
cleaned the completion and created a new list with the current text of 255
chars which normally matched only once.

The buffer size is now set to 512 for all internal used string buffer.
Removed gtk_entry_set_inner_border for gtk3 build (#38).
Fixed too small completion box if completion list is small (#39).

If there where only few item in completion list, the completion list where
shrinked under the allow winheight/3 value (only gtk3). Now we set the min
content height of the scrollable window also if this should already fit.
Merge branch 'treeview-completion'