Basic table navigation and reporting of table information in virtual buffers

[jteh]

This work is being done in conjunction with the new nvVBufLib.

• The new gecko_ia2 and adobeAcrobat backends now include table information in the virtual buffer.
• Information concerning cells that span multiple rows and/or columns is not yet included.
• There is also initial code in NVDA's nvVBufLibIntegration for table navigation and reporting.
  • Table navigation is fine for simple tables, but doesn't handle empty cells very well. I need to rethink this.

[jteh]

nvVBufLibIntegration was merged into main, so this is now in main. The same issues still apply.

[jteh]

For reference, the nvVBufLibIntegration merge was r2803.

The basic functionality works, so I'm closing this as fixed. New tickets should be opened for the remaining issues.