Install button is not deactivated while installing - Ubuntu 16.04

[malintha]

The font installation toggle button is not being deactivated once it is clicked and when the font is being installed. When clicked on it again, font installs twice.

Steps to reproduce:
Click on the install toggle button.
Click on it again while the font is being downloaded.

[heimdallrj]

Thanks! @malintha. Will consider this as a high priority bug, and push a fix as soon as possible.