Make the chat window scrollable with the keyboard and show scroll-arrow-buttons
Currently, running Gajim-Portable-Master-2018-03-29.exe, the contents of the chat window are only scrollable with the mouse wheel or by dragging the scrollbar up and down. In the future the chat contents should be scrollable via the arrow, page up, page down, Pos1 and End keys. In addition to that keyboard control it would also be usefull to show scroll-arrow-buttons on top and bottom of the scroll bar tracks.