feat: Extras, tap-hold, and modifier keys

[ids1024]

Initial implementation of #146.

Backend representation for mod-tap, layer-tap, and modifiers+basic keycode, with conversion to and from text format and numeric keycode.

Add shift-select for modifiers with a non-modifier key, extras tab, and tap-hold section.

Sensitivity needs to be set based on whether shift is held and what is already selected. Getting this to work properly is awkward with how GTK modifiers are handled. More things need to be added to extras, the layout for basic keys needs to be updated for the latest design, and various style details need to be addressed. And it needs to not show these things for internal keycodes.

[ids1024]

I've been busy with Cosmic and Iced related things, and sizing and alignment of the keycode picker has posed a bit of a challenge, but anyway, functionally this should be working. Some things may need to be fixed, and we may want to adjust the style of some things.

To test, it should now be possible to just run apt-manage add popdev:configurator-feature-update, followed by apt update and apt upgrade.

So the problem I've had with sizing and centering of the keycode picker: it's using a custom PickerGroupBox widget to display the picker groups in a way that can reflow depending on available space. The way it determined how much size to request doesn't work so well now with the group for basic keycodes layout out like an ANSI keyboard (wider than previous groups). Also the way it was handling centering only worked for centering the PickerGroupBox, but now it's in GtkStack, etc, and those should be aligned similarly.

I spend some time trying to find a way to make the natural size it requests depending on how it reflows (which isn't really possible; but perhaps that's not the issue, or only part of it). Currently I have it set to allocate the same width as the widest group, which looks alright but seems a bit narrow on wide windows. But allowing it to scale to, say, 1.2 times this just makes it look off center. Unless the individual groups were centered.

[maria-komarova]

Looking at the configurator and after some chat exchange:

• Currently, one can't switch between tabs (Basics, Extras and Tap-Hold) before clicking on the key on the keymap. It might be good to allow people to switch between tabs even if the keycodes are disabled before someone chooses the key.
• Let's try the following order for the keycode groups under Basics: main ISO layout, Navigation, Other actions, Other ANSI/ISO symbols, Controls, LED controls, Media, Layer keys, Numpad. Same order on the Tap-Hold tab.
• The spacing from the view switcher to the content underneath (for example, first keycode row) should be 18px, looks smaller in testing.
• Double check the spacing between "rows" of keycode groups, should be 36px, can't quite figure out if it is smaller than that. Same for steps on Tap-Hold.
• The spacing between the first keycode group (main layout) and the copy "Shift + click..." under it seems too tight. Let's try 12px. Same applies to the "Shift+click..." copy under modifier keycodes under Tap-Hold.
• Spacing between keycodes inside each group in the keycode picker is not consistent. We have 2px on the mockups on all of them. Looks like main layout and Numpad has something similar to this so it would be good to make sure other groups have the same across all tabs.
• When I choose Left Shift for the modifier under Tap-Hold, then choose Access Layer 1, the selection doesn't move to Access Layer 1. It gets highlighted with blue background but modifier function doesn't get remapped. Seems like a bug because it moved after the second attempt.
• I've noticed we have Shift + Click to combine modifiers for Tap-Hold under the main layout keycode group. But it doesn't look like one can actually combine them. We might want to remove the copy on this tab then.
• Let's try using horizontal scroll when the window gets too narrow to fit the keycodes.