ZKL: some visual tweaks to startup menus (for first time user only)

SelectWritableFolder Menu: buttons now adopt current ZKL theme and added "Browse Folder" button (so it won't look different than ZKL and the "Browse Folder" help so people don't need to copy-paste or edit the path string manually).

Program.cs: the message "Startup folder is not writable" is now displayed in Caption section of the error message instead of together with text message but empty caption. (resulting in more cleaner and compact message)