Fix erratic editing behavior

[aerkiaga]

This fixes the regression described at https://github.com/OpenChemistry/avogadroapp/issues/239. This issue appeared with d2dcfae, due to a small change in avogadro/qtgui/rwmolecule_undo.h wherein RWMolecule::addAtom() was made to be passed m_atomId rather than m_atomUid, when it should expect a UID.

The consequence was that, when two carbons were bonded and their hydrogens adjusted, the hydrogens on one of the carbons would be removed and not be added back, due to the passed UIDs actually being their IDs, which happened to be equal to the removed hydrogens' UIDs.

Then, trying to add atoms around this molecule with 5 atoms would pass atom ID 5 as UID, this being the UID of one of the existing hydrogens, which would simply get moved to the clicked location.

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[github-actions]

Here are the build results
Avogadro2.AppImage
macOS.dmg
Win64.exe
Artifacts will only be retained for 90 days.

[ghutchis]

This definitely seems to fix some bugs, so I'm going to merge - and I'll sort out the Ubuntu 20.04 build issue.

[welcome]

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