### Description of the changes:
There are currently 11 copies of the `EMPTY_UNIT` object in the
`Calculator.exe` binary, which not only wastes space/footprint in the
binary itself, but also means that each copy must be separately
initialized, which effects performance.
The reason for this is that the object is defined in a shared header
file, which then is included by multiple .cpp files, causing each
translation unit (.obj) to get a full complete copy of the object.
By marking the object as `inline` we can
instruct the linker to define the object once, as we do
not need to have 11 unique versions of the EMPTY_UNIT object,
we only need 1.
The net result is that the `Calculator.exe` binary size is reduced by
2,560 bytes (or 2.5 KB) with no change in behavior, other than
the small performance benefit of not initializing 10 redundant copies
of the object.
### How changes were validated:
- Manually tested.
