feat(window): use the frameless title bar on Linux (#279)

Linux windows used the GTK menu bar and the system title bar, unlike the frameless main toolbar on Windows.

Linux project and Settings windows now turn off decorations, install no GTK menu bar and draw the main menu and caption buttons in the title bar.

Co-authored-by: xRookieFight <[email protected]>

src/workbench/TitleBar.module.css

@@ -99,7 +99,7 @@
   color: var(--fg-secondary);
 }
 
-/* Windows frameless window: caption buttons sit flush with the right and top edges. */
+/* Frameless window (Windows, Linux): caption buttons sit flush with the right and top edges. */
 .frameless {
   padding-right: 0;
 }