Reduce layout-changes when switching views and make layouts more consistent

An input-device should generally not change shape/functionality during use, unless there is a good reason for that and it is clearly indicated when that happens, as otherwise it will be more complicated and exhausting to use, and learn to use, the input-device.

To improve that, this commit will:

• Make it so that the space-bar, view-switching buttons, backspace-button, and return-button will not move around when switching views.

• Add placeholder-buttons and/or stretch buttons to fill gaps.

• Remove buttons that are duplicated in the same view.

• Add buttons for entering commas, on the base-views of some layouts.

• Make the shape of layouts fit into a rectangular space; usually without empty space.

• Align layouts more closely to the expected aspect-ratios, to reduce changes when switching between languages, and use more of the available space.

Other changes:

• .-buttons are generally not highlighted anymore.

• On the layouts gr and gr+polytonic, the highlighting for punctuation has been switched to subtle-highlight from special, so those can be visually distinguished from buttons with special functionality.

• The Spanish characters ñ and Ñ on the French (Swiss)-layout (ch+fr) were making the layout wider than it should be. As there was no other appropriate place for those buttons, and the layout is meant for entering French, not Spanish, those buttons have been removed from the layout.

Some layouts have been changed much:

• gr+polytonic (which requires more work, but it is much more usable now)
• ir

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comparison_-_reduce_layout-changes_during_view-changes_and_make_layouts_more_consistent

The placeholders can be replaced with useful characters later (in another MR).

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Fixes #312 (closed) and improves #397 .