Fix duplicate results for sessions, simplify and optimize

Summary:
The sessions are now only loaded when the plugin is initialized or the folder changes.
Additionally the option to search for a session just by the name has been removed,
because Kate itself provides this functionality (demonstated in screenshots).

Test Plan:
Create sessions, they should show up like in the last two screenshots.
After testing this create/edit/delete sessions, the changes should be automatically applied.
Before:

After:

Search with "kate" prefix should work as before:

Reviewers: broulik, davidedmundson, ngraham, Plasma, Kate, cullmann

Reviewed By: Kate, cullmann

Subscribers: cullmann, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D27109