Changed, "Select Similar", "Paste Properties Special" actions properties...

Changed, "Select Similar", "Paste Properties Special" actions properties windows, Linedefs: removed "Sidedef Properties" from Linedef flags tab.
Fixed several issues with "Synchronized Things Selection" setting:
- Things were not deselected after dragging a single sector.
- Things were not deselected after editing properties of a single sector.
- Things sector references were not updated after using "Join Sectors" and "Merge Sectors" actions.
- Things (de)selection is now applied to all things, instead of only the ones visible using current Things Filter.