Update layout for pathway reaction on canvas

[yauhen-karmyzau]

Building pathway reactions, we align arrow's starting points vertically:

Pathway reaction layout should be updated to meet the following requirements:

1. Single arrow type should be changed to have the same style head arrow, as we use for multi-tailed arrows.
2. Pathway reaction should be divided into sections and each element should be centred in the section.
3. Margin between sections must be equal to reaction-component-margin-size (distance between an arrow's right/left point and closest bond of the widest molecule in right/left columns).
4. Section width must be equal to the widest molecule in the section.
5. Multi-tailed arrows' and single arrows' start and and points must be aligned vertically in a section as well.
6. Default size of arrows in a pathway reaction must be changed:
   1. Head arrow's line must be 6.5 default sizes of the bond
      

[ivanmaslow]

Tested in scope of Load RDF task, for more details check comments here - #2102, cascade reactions are displayed with correct alignment and filled arrow style.
2 defects are detected, they should be retested after fixes:

• Not correct length of head arrow after loading from RDF or layout action #2453
• Several cascade reactions are not aligned after layout or save/load to/from RDF actions #2498

[ivanmaslow]

New defects related to length are detected:

• Not correct length of Multi-Tailed Arrow and Single arrow after loading from RDF or Layout action #2555
• Not correct length of Multi-Tailed and Single arrows for reactions with atoms after loading from RDF or Layout actions #2583
• Arrows overlap atoms of chemical elements after save/load to/from RDF or layout actions #2586