Various performance optimizations

[JasonG-FR]

• Get the uptime when the app is launched and keep track of the time difference internally instead of polling it every 500ms.
• Stop updating the values in the UI if they didn't change (status, percentage, health) every 500ms.
• Try to read the values directly using Python open (everything is a file after all) instead of subprocess to check if there is a significant performance improvement.
• Try to regroup functions in a module so they can be pre-compiled and check if it improves the start-up speed.

[JasonG-FR]

b59aafa : Only checking battery health once at startup and added support for when the data isn't available (PineBookPro).