Measure holds the value and a unit for CSS properties.

#264

This way elements can handle their logic themselves without having to
rely on outside utility classes.

Also restructure files in common in a more logical way.

Components with bigger scope are put to the component root, canvas, 
props panle and new-element-panel.

Change import paths of CommonModule files in editor and player

This makes newly created elements appear next to each other instead of 
on top.

It is enough to set the row and column values. Spans are now only 
changed from the props panel.

gridColumnStart: number;
gridColumnEnd: number;
gridRowStart: number;
gridRowEnd: number;

becomes

gridColumn: number;
gridColumnRange: number;
gridRow: number;
gridRowRange: number;

Improve code and prohibit resizing of section when child element is 
dragged.

Now uses the implicit grid in auto mode.