Sector Edit form and Linedef Edit form now support float texture offsets.

Tag Statistics form: you can now double click on Sectors, Linedefs or Things cell to select them, and right click to open their properties.
Texture size labels were displayed incorrectly in some cases.
Rewritten VerticesMode.DeleteItem() once again...
Vertex Edit form now works in realtime mode.
Vertex Edit form: ceiling and floor vertex offsets can now be cleared.
Added StairSectorBuilder plugin (I suppose some external plugins will stop working in GZDB because I've changed ButtonStep to float in ButtonsNumericTextbox a couple revisions ago...).
Preferences form: action description is now scrollable.
Changed background color of Sector Edit form.
Vertex' ZCeiling and ZFloor properties are now managed internally.