Add support for opening structures in CDX format embedded into MS PowerPoint

[vanoprenko]

Background
Some Ketcher users need to deal with chemical structures embedded into MS PowerPoint presentation as ChemDraw OLE objects. Users need to select structures from those stored in the presentation and open them in Ketcher.
Supported formats are PPTX for presentation and CDX for chemical structures stored in OLE objects.

Solution
If user selects a .pptx file in Ketcher's file selection window (Open Structure -> Open From File), Ketcher should find all ChemDraw objects in the file and extract CDX content from each object. Structure selection window should further be displayed instead of file content preview.

The structure selection window should include presentation file name (without path), list of chemical structures found and preview frame showing selected structure.

Structures in the list should be displayed in order of their appearance in the presentation file. List items should be named Structure 1, ... Structure < n >, where n is the number of structures found.

Once user selects an element in the list, Ketcher should use corresponding CDX to render the structure in the preview frame. User may further use 'Add to Canvas' and 'Open as New Project' buttons to open the selected structure similarly to regular file opening workflow.

If no chemical structures found in the selected presentation file, a message box "No embedded structures found in the file" should be displayed and structure selection window should not be shown.
When structure selection window pops up, it should have first item in the structure list selected.