mimic QDir::listSeparator() on tag file handling to support Qt < 5.6
speed up tag files loading by deferring duplicates checking
update tag file menu with tag files loaded when opening files
fix separator issue in nedit.tagFile setting
extract version info from git for display on About dialog
support opening files by drag-and-drop from external file managers
add application icon for nedit-ng.exe and nc-ng.exe on Windows
fix errors on loading/unloading ctags file on Windows