🍒applets/taskmanager: Fix applet's layout size with multiple rows

The property Layout.maximumWidth of the taskList didn't take into account the number of rows. This was compensated later when calcualting its actual width, but not for the applet's preferred size, resulting in extra demanded space.

To fix it, move the influence of the rows directly into the property.

This was only noticeable with the settings forceStripes==true, and fill==false because in any other combination, either the space is wanted, or the tasks fills the available space before creating a new row.

Due to symmetry, the same is done on vertical panels for 'Layout.maximumHeight` and the number of columns.

BUG: 490319 FIXED-IN: 6.1.4

(cherry picked from commit 71ddcebf)