Detachable windows (#3532)

Allows detaching a window from LMMS's main window, making things like working on multiple screens easier.

The behavior of detached windows can be customized in the Settings.

Closes #1259

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Signed-off-by: Dalton Messmer <messmer.dalton@gmail.com>
Co-authored-by: Hyunjin Song <tteu.ingog@gmail.com>
Co-authored-by: Dalton Messmer <messmer.dalton@gmail.com>
Co-authored-by: SpomJ <mihail_a_m@mail.ru>

data/themes/default/detach.svg

@@ -0,0 +1,6 @@
+<svg width="17" height="17" version="1.1" viewBox="0 0 17 17" xmlns="http://www.w3.org/2000/svg">
+	<g fill="#fff">
+		<rect x="4" y="11" width="9" height="2"/>
+		<path d="m4 9 4.5-5 4.5 5"/>
+	</g>
+</svg>