If you want to transfer colonists from (click a "ship" icon on) a multi-species colony the "Embark population" dialog is called. If from single-species - just picks a single "unit" (the "old" way). The "old" way has disadvantages - you can't pick more than one in a single "transaction", and the "focus" (the lower third of the screen) immediately switches to the "source" system (very inconvenient).
Is there (or could you make) a way to always/forcibly call the "Embark population" dialog after clicking the "ship" icon?

You can select more than one unit of pop to transfer; just click multiple times on the source location. The number over the lower part of the population icon will increment to show the number being transported.

Thanks for the tip.
However, the major annoyance is the change of "focus". Selecting the destination system and transferring from the planet's list is the only bearable way in a large galaxy (even 80-100 stars, but specially bigger) - works fine with the dialog, doesn't work without (the destination gets lost and hard to find than).

An alternative proposal - could you allow "dropping" colonists to a big planet picture (the upper left part of the screen - it stays in place even while the "focus" moves)?