Inconsistencies with Gtk dialog

[JubbeArt]

Noticed some minor inconsistencies with other Gtk applications that also uses dialogs:

• Cancel button should be on left and not on right
• Accept button always says "Open" or "Save" instead of just "Ok"
• Title is "Open File" and "Save File" or "Save As" instead of just "Open" and "Load"
• The dialog and text when asked to overwrite a file is not standard. Most seems to use dlg.SetDoOverwriteConfirmation(true) which automatically creates that dialog in case of an overwrite

I could open a PR to address these if you want

[sqweek]

Thanks for cleaning this up! I've now merged the PR, apologies for the delay.

I had to resolve some conflicts and apparently my test linux environment still doesn't work with gtk3 for reasons I don't have time to look into, so please let me know if anything doesn't work following this commit.