Simplify StFBVisibleFilter implementation

[astares]

As pharo-project/pharo#15501 has been integrated today into Pharo 12 it is much easier to
test a file reference if it is visible or hidden using the #isHidden method.

We can therefore simplify and

• remove the the OS specific code in StFBVisibleFilter>>#predicate: and use #isHidden in this method
• remove the Windows specific extension methods to FileReference from package "NewTools-FileBrowser"

[astares]

So from

predicate: aFileReference

	OSPlatform current isWindows ifTrue: [ 
		^ aFileReference isHiddenInWindows not ].
	^ (aFileReference basename matchesRegex: '\.[^.].*') not

we can reduce to

predicate: aFileReference
	 
	^ aFileReference isHidden not