[desktop] Use native window decorations on Linux (Wayland) where supported

[IlyaBizyaev]

Currently, Ente's window uses client-side decorations (CSD) on Linux:

As you can see on the attached screenshot:

• There is no strong reason to do so: Ente does not place any custom controls in the title bar, so the system-provided decoration would look more integrated with the desktop environment and would be quicker to use because of the same window control positions as in other applications' windows.
• Ente's custom window decorations are missing shadows: if the window in the background is also white, it's hard to tell where Ente's window starts and ends. This makes it pretty awkward to use except when maximized. For comparison, Google Chrome also supports CSD (optionally), but does draw proper shadows around them. (It is as expected that the window manager does not add shadows to windows that opt into custom decorations)

Expected appearance in e.g. KDE Plasma (edited):

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Potentially useful info:

• Relevant standard: https://wayland.app/protocols/xdg-decoration-unstable-v1#compositor-support
• Solution in LocalSend (another Flutter app): Disable GTK3 header bar by default on Linux and use regular title bar instead localsend/localsend#2360