lomiri: Let shell honour GRID_UNIT_PX variable's value

Follow-up issue for qtmir#5 (closed)

How to reproduce this issue:

• Install Debian 13 minimal on physical hardware (e.g. a notebook) or a VM, and run apt install lomiri. Reboot.
• Login into Lomiri
• Launch an app, e.g. lomiri-system-settings (the cogwheel icon in the launcher's drawer)
• Observe font size, icon size, etc.
• Logout
• Now edit /usr/bin/lomiri-session (via tty) and enforce a GRID_UNIT_PX value of 16 (default is 8)
• Login into Lomiri again.
• Observe font size, icon size, etc. again

What you will observe:

• In Lomiri apps, the fonts and icons will be twice the size compared to before tinkering with GRID_UNIT_PX in /usr/bin/lomiri-session
• But the shell itself (launcher, drawer, indicators, spread, etc.) will not be scaled up

What should happen:

• All UI elements of the Lomiri shell and of Lomiri apps should scale when tinkering with GRID_UNIT_PX.