Add IntelliJ IDEA project files to ignored

Summary: Used by some developers, IntelliJ IDEA has nice git, diff and replace tools.

Reviewers: #plasma_workspaces, #plasma, davidedmundson, broulik, ngraham, nicolasfella

Reviewed By: nicolasfella

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D26925