feat(microdrop/device-view): give "edit electrode channels" input field focus on open

[ryanfobel]

When a user is assigning channel numbers to a new device mask, they have to right click on each electrode and choose the "Electrode/Edit electrode channels" menu item to launch an input dialog:

When the dialog is launched, it's input field does not have focus, so the user must mouse click on the input field before typing in a channel number. Giving this input field focus when the dialog box is launched would streamline the process.

[cfobel]

The input dialog for editing channels is created by the device_info_plugin using pygtkhelpers.schema.schema_dialog().

There are at least a couple different approaches to give the input field focus:

1. Do not use pygtkhelpers.schema.schema_dialog() and instead create a custom dialog with the desired behaviour.
2. Modify pygtkhelpers.schema.schema_dialog() to provide a way to set the default focus, e.g.:
   • By schema field name
   • To first input field (note that order of schema inputs is currently undefined, since dictionaries are unordered).
   • Either?

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Workaround

As a temporary workaround, I would recommend the following mouse/keyboard sequence:

Right-click, `<e>`, `<c>`, `<alt><tab>`, `<shift><tab>`

followed by entering a value and pressing <enter>.