Add option to pull an individual form from disk

[ggalmazor]

This issue is related to #37

This repo has a separate UI dialog that can be built individually to produce the ODK Form Uploader binary, which is used by users to upload big forms to Aggregate.

Our users can't use Briefcase for that because it lacks the option to "install" an individual form directly from their hard-drive.

We could provide a new Pull source option to "install" a form from a hard-drive location and, by doing so, Briefcase could replace the Form Uploader binary, letting us simplify the ODK tool-stack.

My proposed behavior goes like this:

1. The user selects a new "Individual form in computer" option from the "Pull data from" drop-down.
2. A file choosing dialog pops up and the user selects the form file.
3. Briefcase checks the form file and verifies that any required attachment is present.
   • If everything is OK, the form is listed on the forms table (it'd be the only form listed)
   • If Briefcase detects any problem, a dialog with feedback is shown

From this point on, the user can normally pull (install) the form just like from any other currently supported source and push it to an Aggregate instance using the Push tab.