Multiple windows bugs

[dacap]

After #3548 we should start fixing some issues using multiple UI windows:

• Multiple windows appear in the Windows 7 taskbar #2804
• Dialog flickers when it's closed and reopened (UI with multiple windows enabled) #3019
• Windows/dialogs open on wrong display (v1.3-beta7, regression) #3030
• Aseprite window is not minimized/restored correctly on macOS with multiple windows UI (v1.3 beta) #3038
• Selecting "Used Colors/Unused Colors" when pen pressure is enabled will paint in the canvas #3500
• Random tooltip behavior #3501 / https://community.aseprite.org/t/problem-with-the-toolbar/20072
• Quitting the app doesn't focus on the window if there's a warning about saving changes #3542
• [Enabled] UI with multiple windows. Not working properly with tool selection submenu. #3552
• Preview Window (among others) is not capturable when "UI with multiple windows" is enabled. #3675
• [Feature Request/Bug Fix] Closing Preview Window in Full-Screen Preview Mode. #3762
• Inactive windows in multiple-window UI mode pass events to below windows #4561
• Update minimum platform to macOS 10.14.6 #4875
• Multiple monitors with multiple windows option enabled could incorrectly limit the bounds of a popup #5427
• Color selector frequently closes when currently in use #5711 (probably related to i3 WM?)
• etc.