linux: create a hidden window inside the platform

It allows us to receive messages from the dispatcher,
which breaks us out of waiting and lets us execute
main thread runnables as a part of the main loop.

crates/gpui/src/platform/linux/text_system.rs

@@ -76,7 +76,7 @@ impl PlatformTextSystem for LinuxTextSystem {
         unimplemented!()
     }
     fn glyph_for_char(&self, font_id: FontId, ch: char) -> Option<GlyphId> {
-        unimplemented!()
+        None
     }
     fn glyph_raster_bounds(&self, params: &RenderGlyphParams) -> Result<Bounds<DevicePixels>> {
         unimplemented!()