Minor DockWidget bug

This is for me to remember to look into.

Can reproduce on Windows and Linux.

• Tabify the Preview and Table of Contents DockWidgets together in the left dock area
• Make sure the Preview tab is the one on top/active
• Close Sigil
• Open Sigil (ensuring the the Preview tab is on top/active)
• Minimize Sigil
• Upon restoring Sigil, the Table of Contents tab is now on top/active.

You can activate the Preview tab again and it will stay active (through minimization/restoration) for that Sigil session. It's only the initial minimize/restore where Preview loses focus.

After further investigation, Preview really has nothing to do with it. The same behavior occurs no matter which DockWidget is tabified with the Table of Contents DockWidget. The Table of Contents demands to be on top/active when restored from that Sigil session's first minimization.

Not a huge deal at all, but I'd like to track it down.