xbase: add "shift up/down" keyboard shortcuts to shift+j and shift+k

These shortcuts slide the currently selected item up and down the list.
The mouse is no longer needed to do most of the operations in the GUI
now that we have this ability.

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: add more tooltips and tweak usage text

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: make "Ctrl+Enter" apply changes and rebase

Make the rebase editor consistent with the commit message editor by
supporting the "Ctrl+Enter" shortcut to accept and apply changes.

Signed-off-by: David Aguilar <davvid@gmail.com>