main/postmarketos-ui-lxqt: various fixes and usability improvements

This MR will improve the usability on LXQt desktop and fixes various minor issues.

• Move LXQt suite from testing (aports!35533)
• As lxqt-core got renamed to lxqt-desktop, add missing additional programs from older lxqt-desktop to _pmb_recommends.
• Add paper-icon-theme to _pmb_recommends and remove it from depends on pcmanfm-qt aport. See aports!38304.
• Add missing custom icon (#1418, but on LXQt)
• Add some bluetooth GUI program by default. bluedevil should be ideal.
• Add onboard by default. Also, start it automatically when logged in.
• Set a bigger window frame (like XFCE does)
• Dark theme by default on panel (like Ambiance but changing inner color from orange to deep gray)
• Add alsa-plugins-pulse and alsa-utils
• Add some other improvements, inspired from !2506 (merged) .

Closes #2142 (closed) .