feat: Allow using native window decorations/titlebar

[ethanc8]

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Is your feature request related to a problem? Please describe it

In versions before v0.5.x, Jan used the system's native window decorations (titlebar, window borders, shadow behind windows, etc). However, in v0.5.1, Jan draws its own titlebar and does not implement things like shadows behind the window or the platform-native right-click menu, at least on Linux with the KDE desktop environment. This makes Jan feel unnatural and annoying to use, as the lack of shadows behind the window cause the window to blend in to other windows, and it generally does not feel like a normal window.

Describe the solution

As a bare minimum, an option should be added to the appearance settings to allow the native window decorations to be used. If possible, we should replace Jan's custom titlebar with the Electron menubar (File, Edit, Help...) when native window decorations are being used. Another thing we could look into is improving the behavior of the nonnative window decorations, such as implementing drop shadows and right-click on the titlebar. This is something that all Electron apps suffer from, while Gtk apps don't suffer as much because the Gtk theme typically implements behavior close to native for Gtk apps that draw their own window decorations.

Teachability, documentation, adoption, migration strategy

On some platforms, the titlebar and window borders of the Jan main window may feel out of place or odd. If you are bothered by this, you can go to Settings > Appearance > Window Decorations, select "Native Window Decorations", and reopen the app.

What is the motivation / use case for changing the behavior?

This behavior is a regression from v0.4.x. It also makes the menubar harder to access.

[sndurkin]

+1, another useful feature of having native window decorations is that in Windows, you can use hotkeys to move the window between desktops and maximize it, and this seems to be lost in v0.5+.