add - files preview architecture along with text file previewer initialization
add - refact FileList to use a new FileListModel class
add - add Icon enum to separate loading icon image files

refact FileList inner methods structure
ref - rename methods in FileBrowserMenuBar with build- prefix instead of init-
add - todo comment in FileList
fix - fire contents changed event in FileListModel
add - AbstractListDataListener which is an empty implementation for javax.swing.event.ListDataListener

i wonder why there is no such implementation in the base package
add - file browser toolbar with two buttons

show/hide hidden files toggle-button
go up one folder button
Merge remote-tracking branch 'origin/master'

Conflicts:
	src/main/java/com/example/FileList.java
	src/main/java/com/example/FilePreviewBuilder.java
	src/main/java/com/example/FilePreviewFactory.java
	src/main/java/com/example/TextFilePreview.java
	src/main/java/com/example/TextFilePreviewBuilder.java