Fix Issue #3: The sidebar hides the main panel
If it is not possible to load the window state (for example, it is the first time that the application runs), set a default value (33%) for the sidebar width.
This fixes Gitlab issue #3 (bugzilla #788621) which was causing the sidebar to extend to the whole window (sidebar_width = 0, i.e. 100%), thus hiding the main panel and rendering the application basically unusable when the sidebar was enabled.
The second version of this patch introduced further fixes to the window resizing code (the sidebar panel is correctly resized upon window resizing or window minimization/maximization).
This third version includes support for the correct migration from the old window state format to the new format (sidebar width as a percentage instead of pixels).