"Electrum" application name should be a constant

[JeremyRand]

Currently, the UI string "Electrum" is hardcoded in a very large number of strings scattered across the upstream Electrum codebase, which makes maintaining a fork with proper rebranding quite a bit more annoying than it needs to be. It would be useful if the application name were stored in a single constant instead.

This should be brought up upstream, but worth tracking in Namecoin's issue tracker since it's directly impacting us.

[JeremyRand]

Note that upstream requires that such a PR not impact i18n. I.e. the naive approach of substituting a variable inside localizable strings will not work, since that will break the i18n scripts.