add - zoom buttons to image preview
ref - make ImageFilePreviewBuilder package local
imp - focus file list after clicking toolbar buttons
imp - center new tab dialog over pane
imp - prompt the user for the new tab at the app start
imp - fix new file list dialog size
imp - do not dispose new tab dialog if list creation failed
ref - insignificant code style fixes
imp - go up a file tree on backspace key
update README.md