qutebrowser: Make xdg-utils optional

Replace all hard-coded instances of using xdg-open with at least one
non-xdg alternative. Mostly falling back to either mimeo or a custom
open script.

qutebrowser/data/userscripts/qute-gemini

@@ -21,6 +21,7 @@
 # continue surfing like normal, only that you can now also access
 # any gemini pages as if they were part of the normal http protocol.
 
+import shutil
 import cgi
 import html
 import os
@@ -379,7 +380,10 @@ def open_url(url: str, open_args: str) -> None:
         with open(fifo, "w") as qfifo:
             qfifo.write(f"open {open_args} {to_open}")
         return
-    os.system(f"xdg-open {to_open}")
+    if shutil.which("mimeo"):
+        _ = os.system(f"mimeo {to_open}")
+    elif shutil.which("xdg-open"):
+        _ = os.system(f"xdg-open {to_open}")
 
 
 if __name__ == "__main__":