This is in Stable v1.64 now.
Go to View/Appearance/Align Panel/Justify
or use the Layout Control icon on the upper right mentioned below.
Other options for the Panel alignment are center/left/right
.
Note that that the Layout icon on the upper right was recently added with this setting:
Workbench > Layout Control: Enabled
Also, as of v1.75 this can be done via the context menu for the Panel. Right-click on an open space of the Panel to the right of the View headers to open the context menu: