Install button still actionable when snap install in progress

[Marco Boneff]

When installing a new snap from gnome-software, the "install" button remains clickable while installing is in progress. If you press the button once again, the store returns an error "Impossible to install «%snap_name%»: snap "%snap_name%" has "install-snap" change in progress".

Test Case
1. Open GNOME Software
2. Select a snap and click on the install button
3. Click again on the same button

Expected result:
- When the button "Install" is clicked once, it should become inactive (change color and not be clickable anymore).
- the label on the button should change showing something like "install in progress"
- (Desired, not forcefully required) A progress bar should indicate that something is happening

Observed result:
Install button is always active and clickable.
When the "install" button clicked , gnome-software returns the error "Impossible to install «%snap_name%»: snap "%snap_name%" has "install-snap" change in progress