tkui: More UI work.

* Separate model-panel (tree&edits) in a class.
* Slider for log-panel.
* Fix to run also in Python-27.
tkui: LogPanel fully functional.

* logpanel: Add popup, filters, threshold, title.
* logpanel: Intercept Tk-exceptions.
* Drop threading; Tk wants always MAIN thread.
tkui, test: Do not work in Python 2

* tkui: Add img in About, add menubar.
* tkui: Buttons outside of ModelPanel.
* build: Depend on Pillow (for imgs).