qt/main: Better file-existence checking within OnMenuRecentFile() and UpdateUITheme()

In OnMenuRecentFile() we don't need to construct a QFileInfo instance
just to check if a file exists, we can just use the static member
function to do that (which Qt's documentation also notes as quicker than
constructing an instance).

In UpdateUITheme(), we just want to try and open the file and check the
success of that operation. Technically speaking, between the existence
check and the open call, the file can be deleted or moved, but still
appear to succeed in code. i.e.

1. Existence check -> Returns true
2. File is moved/deleted
3. Open is called, the return value of which isn't checked
4. Nonsense behavior

This way we combine the existence check and the open into one.