Create format code for height with decimals

There are currently two in-game windows that use height properties. The first is the Cut-away View window (ViewClipping.cpp), which uses it to display the vertical cut height. The second is the Map Generator window (MapGen.cpp), which display several height properties used in the map generation process. Both implementations are different, and neither use the {HEIGHT} format code.

Both windows should be made to use a new format code to show height with two decimal places, e.g. {HEIGHT2DP}. This new format code should take the user's preferred units into account (metres, feet, in-game units).

An implementation of this issue should tick the following boxes:

• Create new HEIGHT2DP format code.
• Recycle the 'suffix' strings from the ViewClipping window (STR_UNIT1DP_NO_SUFFIX, STR_UNIT1DP_SUFFIX_FEET, STR_UNIT2DP_SUFFIX_METRES)
• Use new HEIGHT2DP format code in ViewClipping window
• Use new HEIGHT2DP format code in MapGen window

Read the current height handling in ViewClipping.cpp and MapGen.cpp, comparing how each window formats and displays its height properties. The work is done when HEIGHT2DP exists, reuses the named suffix strings, and is used by both windows while respecting metres, feet, and in-game units.