main.view: rename "Action" to "Actions"

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: allow resizing the diff/rebase widgets

Use a splitter to make the UI more flexible.

Signed-off-by: David Aguilar <davvid@gmail.com>
xbase: auto-select the first item on startup

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