Minor FolderManager code improvements

[jeremypw]

• Although unlikely for local files, FolderManager.File objects are not guaranteed to have a non-null FileInfo - this is now checked before dereferencing in is_valid_directory and icon.
• No use is currently made of the ability to suppress hidden files in the folder (they are shown de-emphasised instead) so associated code is removed
• is_valid_directory and is_valid_textfile are now both properties and are only determined once to improve performance.

[ryonakano]

Sorry for my late review, another tiny thing and otherwise looks good.

[jeremypw]

@ryonakano Thanks for review! I've fixed that obviously unnecessary repetition.