Add PerforceImportMetadataWidget.

Not all functionality is implemented in this version. A subsequent Differential will contain more (maybe the rest) of the functionality.

The widget makes some very basic tests to
a) See if the perforce client executable is a valid one
b) Test if the specified repository server url is valid
c) Test if the repository contains client workspaces

This patch is dependent on: https://phabricator.kde.org/D14372

and reports to the user if any of the tests fail

In order to test it one needs to:

1. Make sure that P4CONFIG is exported to the environment KDevelop is running in
2. Make sure that Perforceplugin is not disabled