System sensors

[vsalvador]

What the PR is about:

Current sensors doesn't include system sensors like motherboard temperature o fan. This PR tries to fix that by letting you see stats (like temp, or fans) for each system sensors in the themes.

What it does:

Getting the data: I've refactored the sensor code (sensors_python.py, sensors_librehardwaremonitor.py) so that it can grab stats from system fan and temperature. Also, changed sensors for fans distiguishing between fan speed (RPM) and fan percent..

New Theme functionality: Themes can now use a new format. I added a theme called 3.5inchThemeSystem as an example.

Keeping old themes working (Backwards Compatibility): Endeed, current cpu fan speed will change from default percentage to rpm speed as the current FAN_SPEED is now returning RPM speed for fans.

Totaly open to any change or refactoring proposal you can have.

Testing:

I've tested this on my setup:

OS: Linux (Fedora 42)
Python: 3.13.5
GPU: Radeon RX 9060 XT
Display: 3.5-inch Revision A
Sensor Modes Tested: PYTHON, STATIC, STUB

[vsalvador]

I can change all themes updating FAN_SPEED to FAN_PERCENT, but before doing this work I want to be sure this request is going to be merged