* Helper for whether a slider is vertical

* Only accumulate delta for the current slider's direction

* Slow down horizontal cursor movement while ctrl is held

* Refactor: Implement the space key as an editing action

The space key is no longer a CHAR key but now an EDITING key. This
allows changing the behavior of the space bar while allowing custom
layout users to define a key outputing a space as before.

KeyModifier and ComposeKey are updated to treat the space key as before.

* Enter the best suggestion when the space bar is pressed

* Refactor: Implement backspace as an editing action

* Undo suggestion when delete is pressed

The delete key (backspace internally) undoes the suggestion if the last
action done was entering the suggestion with either the space bar or the
candidates view.

* Add an option to enable space bar autocomplete

This option is off by default. Undoing a completion with backspace works
in any case.

* Refactor: Track selection emptyness in CurrentlyTypedWord

This removes an expensive RPC call.

* Refactor: KeyValue constants

Add constants for keys that are accessed from the app's code. This
simplifies the fake pointer handling in Keyboard2View.

* Refactor: Pre-compute action key replacement

The action key and the enter swap are pre-computed in EditorConfig. This
simplifies the code.

The "Switch to the last used keyboard" is renamed to "Switching between
input methods" and allows to select "Switch to last used", which was the
default in older releases.

* Add the small capital letters dead key

It can be added to the keyboard from the settings or into custom layouts
as `accent_small_caps`.

Add all the symbols listed on https://en.wikipedia.org/wiki/Small_caps

* refactor: Better printing for KeyValue in tests

* Fix parsing of escaped characters in macros

The parsing code was bugged with custom macros like `symbol:\abc` and
`symbol:\\abc`.

* Selection mode: Space to cancel the selection

This adds the "selection mode", which is activated when text is selected
in the text box. The selection mode is exited when the selection is
cleared.

While the selection mode is activated, the Space and Esc keys are
modified into the "selection cancel" key, which remove the selection
without changing the text.
The space bar is otherwise easy to type by accident during a selection
and causes the selected text to be deleted.

* Selection mode: Move each ends of selection separately with slider

When the selection mode is activated, the space bar sliders change how
they affect the selection:

- The left side of the slider moves the left position of the selection.
  To shrink the selection from the left side, the slider must be
  activated by sliding to the left, extending the selection
  temporarilly, then by sliding to the right.

- The right side of the slider affects the right position if the
  selection.