- addresses issue commented in #745 (review)
- when panel maximized, buttons are displayed even if viewer is not in render path (see #744 for an explanation of that feature). This is not 100% satisfcatory, but better than before IMHO
- minimized() and maximized() were originally two separate signals (in NodeSettingsPanel), so I kept it that way, but we could have simpler definitions of NodeGui::onSettingsPanelMinimized() and NodeGui::onSettingsPanelMaximized() that simply call NodeGui::onSettingsPanelClosed(true) or NodeGui::onSettingsPanelClosed(false).

- maximize properties panel when double-clicking a node in the nodegraph
- do not show the viewer buttons if a node is selected in the nodegraph but has its properties panel minimized