fix: Unsatisfactory "Close To Tray" behavior on macOS

[shenlebantongying]

goldendict/goldendict#1356

I think I get the problem now.

The mainWindow is hidden() but never show() again. For macOS, there is no mainwindow anymore, thus clicking the dock cannot bring it back.

Changing this line to showMinimized() solves the problem, ⌘ F4 toggling will work.

https://github.com/xiaoyifang/goldendict/blob/8ffa1f66436ed13368d6e823abd86455e25afd21/mainwindow.cc#L2925-L2927

The hack is for win/linux in which the taskbar icon will disappear if a window is hide() but the tray is still there. We cannot do the same on macOS.

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CRAZY FACT: @Arnie97 knows the reason since the beginning, wtf.