Sanitize pro1 xkb layouts and add a us-intl variant

[Kabouik]

This is an attempt to sanitize the file by removing unnecessary or erroneous definitions, and it adds another us variant as well as some comments on how to use it.

In the original pro1 file, lines 1 to 11 corresponded to a partial us layout to change p and l third level into / and ?. However, this has become obsolete because those changes are also included in the us-international layout starting at line 77 (of the original pro1 file), which has become the default when selecting "English (US)" in SFOS Settings > Text input > Active layout.

What this commit does:

• remove the partial us layout since it the default us international layout does the same;
• all variants were defined as "partial", including those that were full layout rewrites, so this was removed for them;
• some changes in formatting and in the order in which options and names appear to better conform to what seems to be the standard in /usr/share/X11/xkb/symbols/;
• fix the name of the default us layout (altgr dead key), there was a confusion with the dead key variant, and comments to explicitly mention that it adds extra characters compared to typical us layout;
• add a dead key variant similar to the default, but moving some dead keys to first and second levels, like the typical us-intl with dead keys;
• add some comments on how to enable several concurrent variants and toggle between them dynamically using Alt+Space. This is not the ideal place to add detailed comments, but the SFOS UI does not allow selecting concurrent variants and toggling between them, so this should definitely be documented somewhere and here doesn't seem too bad.

I have tested it a bit in all the layouts we came up with so far, I believe the functionality is conserved: default de, ru or us layouts are correctly selected when picking them in SFOS Settings > Text input > Active layout. Layout toggle between ru and us still works as expected when running the appropriate dconf commands to enable this option, and now the extra us variant with dead keys can be selected in the same way, as described in comments. Note that using dconf to enable two concurrent layouts and the toggle option will clear the selection in SFOS Settings > Text input > Active layout, and selecting one again from the UI will disable the dconf options. No need to restart Lipstick, it seems.

Let me know what you think, and please test if you can. I'm happy to try improving this.

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Also, one question remains: now that / and ? are on the Sym key by default, should we keep them on p and l third levels too, or should we restore ö and ø that are normally sitting there on a us-intl layout? Both choices have their pros and cons. Keeping / and ? fits what is actually written on the keyboard, which makes it easier to understand for new users, and more straightforward, but this means ö and ø are harder to use (they can still be used, but they're one level higher, and their capital letter version requires using Caps Lock), meaning it is less easy and less straightforward for those writing in a Scandic language.

[piggz]

@NotKit im not comfortable enough with keyboard configs to comment on this, how does it look to you?