Make file selector strings translations less error prone.

Segregate translatable messages and file extensions in strings given to
wxFileSelector. This is to avoid possible typos in translations changing
the behavior of the application, and to save the translators some
trouble with long and cryptic messages.
Use consistent file types naming.

Use the same, less obscure, names for file types with wxFileSelector
that are already used for filetype information for the OS.

Also standardize on the correct (for both OS X and Windows) spelling,
with title case.