DT-954: add percentage support

[sergio-costas]

This MR adds two new DBus methods that allow to set a percentage value in the progress bar, or set again the pulsing bar.

This will be useful to give to the user more info about how the snap download is going.

[hellsworth]

TLDR; This change looks great to me!

I've built and installed this snapd-desktop-integration. Note that to install it I needed to set experimental.user-daemons=true.

$ sudo snap set system experimental.user-daemons=true
$ sudo snap install snapd-desktop-integration_0.1_amd64.snap --dangerous

Then using d-feet, under the "Session Bus", I was able to send strings to a few different key components:

• ApplicationRefreshCompleted: "firefox", "", {} and we see the pop-up showing a downloading string with the bouncing blue bar

• Without closing the pop-up window, open a window for ApplicationRefreshPercentage: "firefox", "downloading", 0.3, {}. After clicking Execute, we see the bar update to reflect the correct percentage (at least visually). I tried this with a few different values and they all looked good.

• Again, without closing the pop-up window, open a window for ApplicationRefreshPulsed: "firefox", "pulsing now", {}. We see the blue bar start bouncing back and forth again and the string has been updated.

To close the pop-up, I opened a window again for ApplicationRefreshCompleted: "firefox", {} and Execute makes the window close as expected.