[cosmetics] [trivial] Extraneous slash in front of defaultSettings.py

[eumagga0x2a]

STR:

• In Avidemux: "Edit" --> "Save current settings as default" and restart the application
• Look at the list at "Recent" --> "Recent Projects"
• Load a video, restore your encoder settings: "File" -->"Project Script" --> "Run Project" and point the filepicker to ~/.avidemux6/defaultSettings.py.
• Repeat Step 2

Actual Results:
There are two entries for defaultSettings.py, the first one with a single slash in front of the file name and a second with a double slash.

Expected Results:
There is still only one single entry for defaultSettings.py with a single slash between .avidemux6 and the file name.

The extraneous slash originates from avidemux/common/gui_main.cpp:916. Is there a reason to preserve it? Maybe some Mac weirdness I'm not aware of? It is redundant on Linux.

A trivial patch is attached.

(".py" added to the file name in Step 3)