Extend operability of xkb panel plugin (adding at least level3 modifier) and improve UI

Submitted by Iko..@..dex.ru

Assigned to Azamat H. Hackimov

Link to original bug (#10091)

Description

Now I've fine useful and necessary to use some basic xorg input features (currently it is level3 modifier). Compose key, available via xkb-plugin (version 0.5.4.3) settings is not enough, currently I make testes with setxkbmap and find some incompatibility: Just setting (adding) a modifier using setxkbmap, for example: setxkbmap -option 'lv3:win_switch,misc:typo' is inoperable.

Complete reset options, for example setxkbmap -option -option 'grp:ctrl_shift_toggle,compose:menu,lv3:ralt_switch,caps:hyper,misc:typo' works ok (xkb-plugin correctly displays layout, layout switch also work fine).

But opening xkb-plugin setting window resets unsupported by plugin xorg server options. Which behaviour is not always wanted.

I suggest to change xkb-plugin settings window, dividing it into three tabs: Basic (containing keyboard type, keyboard layout management, switch layout, layout manage type, and add option reset xorg options on opening settings window), View (containing display style and type) and Advanced (containing Compose modifier, level3 modifier and maybe something else, for example level5 modifier), maybe with some help messages, because Compose key logic significantly differs from shift/level3 one.

P.S. I find an issue in mentioned version of xkb plugin: "Edit layout" button works as "Add layout"

Version: 0.7.1