fix(developer): run kmlmc from Keyman source path

Makes it possible to run the kmlmc.cmd compiler wrapper script from
Keyman Developer while debugging from source.

Adds a new class `TKeymanDeveloperPaths` which is similar to
`TKeymanPaths`, to provide a future single location for all Keyman
Developer path-related functions. There are a number scattered through
the source at present, e.g. in `RedistFiles.pas`, so this is just a
starting point.