loadLibrary: Use enum values to define what type of plugin we load

Summary: Enum values are more lightweight and easier to compare than strings.

Test Plan:
The manual test ".../kross-interpreters/build/tests/krosstest
.../kross-interpreters/tests/testguiform.py" should open a dialog. It validates
both kross interpreter loading and kross module loading.

Reviewers: Frameworks, aacid

Reviewed By: aacid

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D19498