Add option in meson to install helper application

Added two new options in meson that allow choosing whether the helper applications (gplugin-gtk-viewer and gplugin-query) are going to be installed or not

Testing Done:
Compiled a few times switching the two new options between true and false and verifying that the behavior was correct.

Bugs closed: GPLUGIN-129

Reviewed at https://reviews.imfreedom.org/r/159/