device-nokia-n900: various fixes and improvements to the user experience in i3wm

These changes are aimed at minor improvements and fixes to the user interface in i3wm:

1. Fixing WiFi signal quality in i3blocks (DvaMishkiLapa/pmaports@e3a9b867).

   /proc/net/wireless no longer exists in the system. I don't know exactly when this happened. On my device it disappeared when I switched from kernel 5.* to 6.*. Now you can get the same information from nmcli.

2. Added battery temperature information to i3blocks (DvaMishkiLapa/pmaports@e7c5eca2).

   I think this is useful, so you can see at least some information about the heating of the device.

3. Added volume control in alsamixer using the volume buttons via i3wm (DvaMishkiLapa/pmaports@a060096b).

   I tied the volume buttons to amixer set PCM 5%+ and amixer set PCM 5%-.

4. Added hotkeys to create a screenshot with imagemagick via i3wm (DvaMishkiLapa/pmaports@35007eae).

   I wrote a simple script screenshot.sh that uses import from imagemagick and creates a screenshot of the entire display in the Screenshot folder in the user's home directory. This script also takes as parameters a different path to save the screenshot, as well as the display number of the X server. This is tied to Shift+Ctrl+p.

5. Added hotkeys to change the brightness of the display backlight and keypad LEDs via i3wm (DvaMishkiLapa/pmaports@96829360).

   I have written simple scripts screen_brightness.sh and kb_brightness.sh that take as an argument the brightness intensity offset for the display and for the keyboard LEDs. Although these scripts are almost identical, I decided to separate them so that each of them would be responsible for its own function. This works through a separate mode "brightness" where w and s are responsible for the brightness of the display and e and d are responsible for the brightness of the keyboard LEDs.