Add the ability to save information about the coordinates of the Stereo flags in * .ket

[Svetlana-Asinovskaia]

1. User creates a structure on the canvas, adds a stereobond (stereo flag appears next to the structure), saves the structure in ket.
   Result: As user hasn't moved the stereo flag the information about the coordinates is not present in ket file and their values are set by default.

2. User creates a structure on the canvas, adds a stereobond (stereo flag appears next to the structure), moves the stereo flag (to a new position or it is returned to the initial position after being moved), saves the structure in ket.
   Result: Stereo flag coordinates are present in ket file. After opening the saved file the stereo flag appears at the position in which it was before saving.

[Svetlana-Asinovskaia]

Check the following scenario of overlapping the Stereo flag with a structure:

1. Launch Ketcher.
2. Create a benzene molecule using the Templates tool.
3. Add a stereo bond (e.g. Single Up bond) to the following carbon atom:
   
4. Add another stereo bond to the following carbon atom:
   

Expected result: Stereo flag is not overlapped with a structure.
Actual result: Stereo flag is overlapped with a structure.

[AleksandrPokrovskii]

Stereo flag is overlapped with a structure.

1. Launch Ketcher.
2. Create a benzene molecule using the Templates tool.
3. Add a stereo bond (e.g. Single Up bond) to the following carbon atom:
   

3)Add a single bond to the following carbon atom:

Expected result: Stereo flag is not overlapped with a structure.
Actual result: Stereo flag is overlapped with a structure.

[AndreiMazol]

It will be fixed in scope of a separate issue: #602
cc: @AleksandrPokrovskii