Add a combobox to choose device name from the toolbar

Currently only allows to edit the text directly, doesn't
support saving the last used device or displaying list of
suggested devices.
Add a chooser for the DMM model

Currently it'll only display UT60E for the normal
builds. but for debug ones it'll also include dummy
substitution DMM which is handy for debugs.

Also added a DMMFactory class to deal with all this
DMM registration/creation.