Exclude .gcode and virtual machine files from indexing consideration

Summary:
gcode files are the CNC machine/3D printer equivalent of build files and contain no
data that is useful to index.

The excluded VM-related files are huge, frequently-changing,
and contain no indexable data.

Test Plan:
Baloo no longer wastes time and kernel watches trying to index my gcode and Virtualbox
disk image files. It also no longer notices every single change to the virtualbox files while
a VM is being used.

Reviewers: bruns, Baloo

Subscribers: pino, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D20983