Fix argument null exception if no icon could be extracted

MemoryCache throws an argument null exception of the value to be inserted is null.
MemoryCache throws an argument null exception if the value to be inserted is null

An empty BitmapImage is created to avoid a slow down, because otherwise ExtractIcon()
would be called on every load. At the same time we get rid of the _triedToExstractIcon
variable.
Show version number from the assembly in the about dialog
Update change log for 0.7.1
Bump version number to 0.7.1
Use assembly version in Inno Setup