Added Draw Grid mode.

When Draw Rectangle or Draw Ellipse mode is enabled, it's settings are now shown in side panel.
Draw Ellipse mode: increased maximum number of sides to 512.
Changed the way tags are shown in Tag Selector controls. Tags without labels are now shown as a number (not "Tag N"), and tags with label are now shown as "N (label)", not "label (N)".
Tag labels are now shown in Thing, Sector and Linedef info panels.
Once again changed the way things are rendered while dragged.
Rearranged the label in PairedIntControl and PairedFieldControl (it is now behind numeric controls).
Optimized MapSet.GetSectorByCoordinates().