Tree horizontal scrolling

[kynikos]

Currently the database tree never shows horizontal scrollbars, which should be instead shown when:

• the descendants of an item are expanded to such a deep level that they have to start beyond the right edge of the tree window
• an item has a heading (plus any icons) that is longer than the width of the tree window

DataViewCtrl only shows a horizontal scrollbar depending on the width of the column header(s), not the content in the rows. For this reason the column header width should probably be set every time an item is added, updated, removed, expanded or collapsed, using http://wxpython.org/Phoenix/docs/html/SettableHeaderColumn.html#SettableHeaderColumn.SetWidth

Also consider ellipsization as an alternative. The standard http://wxpython.org/Phoenix/docs/html/dataview.DataViewRenderer.html#dataview.DataViewRenderer.EnableEllipsize cannot be used because the custom renderer draws the text in its own special way.