Designer: Can't find property index file

To reproduce:

1. Open Designer
2. Click "Help"
3. Click "Contents"

This results in an error complaining about being unable to find the property index file. The correct files need to be installed and update the source so that it can be found.

Comment copied from !7 (merged)

Alec:

In checking this out I uncovered another possible bug. It appears Designer is looking for some Qt documentation that isn't installed now using Meson. doc/html/* isn't installed and so Designer can't find propertyindex. On a default installation it fills it with /opt/osiris/share/designer/doc/html in the Preferences dialog. That isn't correct apparently, because propertyindex shouldn't be in designer/doc/html anyways, as it's in the Osiris(Qt) documentation. From my look through the code, I think it originally just copied doc, and OSIRISDIR was set to doc/html/designer, then to set the default documentationPath it would get the parent directory (which in this case is doc/html). That behavior is not correct anymore, in addition to the html documentation not being installed.

To make it short, need to install doc/html into something like <PREFIX>/share/osiris/doc/html, and then update the default value that documentationPath() provides. I believe this is set in help.cpp around line 88. I could be wrong on that however. Let me know if you see the same thing.