Rule Editor: Fix handling of window resize

[JulienCochuyt]

The Rule Editor has undergone several big changes, namely for Multi-Criteria support and Overridden Properties (WIP #21).
Part of the handling of window resizing has been broken.
As a rule of thumb:

• The selected pane should remain in its allotted width, avoiding horizontal scroll bars as much as possible.
• Whenever possible, the selected pane should fit within its allotted height for the default window height.
• Care must be taken to ensure all control are properly scaled for different Windows display scaling settings.

[JulienCochuyt]

Fixed as of f654e6e, 619513c and 24e005b on the branch for PR #21