Always use standard QT color dialog on Linux

On Linux only Gnome uses its own color dialog, and it's inferior to QT's own: See the same issue @ https://github.com/OpenShot/openshot-qt/pull/2425

(This is/was visible only on some distributions' packaged versions of Anki, not via ./run.)

isLin check since QT on Windows already uses the default, whereas Mac's own picker seems superior (from what I can judge by screenshots).

qt/aqt/editor.py

@@ -23,7 +23,7 @@ from anki.hooks import runFilter
 from anki.httpclient import HttpClient
 from anki.lang import _
 from anki.notes import Note
-from anki.utils import checksum, isWin, namedtmp, stripHTMLMedia
+from anki.utils import checksum, isLin, isWin, namedtmp, stripHTMLMedia
 from aqt import AnkiQt, gui_hooks
 from aqt.qt import *
 from aqt.sound import av_player, getAudio
@@ -665,6 +665,11 @@ to a cloze type first, via 'Notes>Change Note Type'"""
 
     # choose new colour
     def onChangeCol(self):
+        if isLin:
+            new = QColorDialog.getColor(
+                QColor(self.fcolour), None, None, QColorDialog.DontUseNativeDialog
+            )
+        else:
             new = QColorDialog.getColor(QColor(self.fcolour), None)
         # native dialog doesn't refocus us for some reason
         self.parentWindow.activateWindow()