Turn history window into dockable widget.
Switched to using .ui file for MainWindow.

As the MainWindow contents were getting complex it was
time to switch to Qt .ui files. This commit adds a
main_window.ui file for the MainWindow as well as the
code generated by pyside-uic in ui_main_window.py .

The main window contains a tab widget for open circuit files
as well as a menu bar and, at least for now, replaces
history_window and history_dock_widget as their custom
behavior was so minimal it made more sense to have them
defined in the main_window.ui file and use promotions for
the sub-widgets that actually have custom behavior.