Show warning if file with same filename exists

Hello. When testing encode bitrate quality a while back, I found that when exporting, if you use a filename that already exists as a file (for e.g: exporting a recording multiple times), Hosaka will let you overwrite it without any warnings. This may be annoying in cases where you’re testing out export settings, or maybe export multiple clips from the same recording.

Should note that if you use the file-picker, it warns you properly, but would be nice if Hosaka itself gave that warning instead