Fixes#260
Description of the changes:
prevent UnitConverterViewModel to reset values when users click on update rates.
recompute UnitConverter's caches (m_ratioMap and m_categoryToUnits) once rates are updated (but check first if the user did/didn't change the category)
How changes were validated:
Manually tested with fake currency rates (HTTP responses modified on the fly via FiddlerCore)
Verified that it works no matter the selected field (From or To)
Verified that the currencies selected are kept after a refresh
### 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.
