[feature-request] Show time elapsed on battery since last AC disconnect

Submitted by Goran

Assigned to Ali Abdallah @ali

Link to original bug (#15805)

Description

Currently, the power manager displays the time remaining on battery, but it does not display the time that has elapsed since the last AC disconnect, which the users might be interested in as well (for instance, to benchmark their battery performance), and having something like text of the elapsed time, in the tooltip of the panel plugin, or when clicking on the plugin, would be nice to have. Now, while the upower interface doesn't seem to provide this info directly, it does store AC connect/disconnect events, see for instance https://askubuntu.com/a/833983, so this is likely a doable task. One potential issue is how system suspends (to RAM/drive) should be handled. For example, a user may suspend his computer to RAM and then bring it back up a couple of days later; the plugin would then show that the system has been on battery for a couple of days, which may be a bit misleading.

Version: 1.6.3