Add automated tests for pdf-roll functionality:
- pdf-roll-page-to-pos utility function
- pdf-roll-page-at-current-pos with various positions
- Customization defaults
- Symbol properties
- Minor mode keymap existence

Fix bug in pdf-roll-margin-color defcustom where the :set
function didn't actually set the variable value.

The registerv-make function was deprecated in Emacs 27.1. Replace it
with a proper cl-defstruct (pdf-view-register) and cl-defmethod
implementations for register-val-jump-to, register-val-describe,
and register-val-insert.

Add tests for the register API used by pdf-view:
- pdf-view-registerv-make-creates-register: verifies register creation
- pdf-view-register-val-insert: verifies insert method works
- pdf-view-position-to-register-stores-position: verifies store/retrieve

Fixes: #333

Make selection-style configurable via `pdf-view-selection-style`. A
default value of `word` might be desirable, as selection is usually
intended to select entire words is perceptually faster than
glyph-based selection.

This affects selections and highlights, but not search.

Limitation:
The selection type is fixed for all currently selected regions.
Selecting multiple regions with different selection styles at the same
time would require storing the selection style for every region and
replacing the protocol for `cmd_addannot` with a more flexible one,
e.g. the keyword-based one used by `cmd_renderpage`.

Closes: #73

This is dead-code for supporting Emacs <26.3

Relates to: #26

* Use `--run` flag to execute nix-shell

The `--command` flag is only required to spawn a interactive shell.

* Do not require gcc to build with nix-shell

`nix-shell` already sets CC with a C compiler, for instance Clang on darwin.

* Remove outdated require in test-helper

`f.el` is not needed anywhere.

- Remove :group arguments from `defcustom` declarations
  - `defcustom` picks up `defgroup` names from the same file, so
    specifying the group is not necessary
- Ensure that all function-names are properly quoted using #'
- Add an explicit header-option setting lexical-binding to t
- Ensure that cl-check-type uses `satisfies` for filenames
- Replace all `defadvice` with `advice-add`
- Replace `(list ...)` patterns with `\`(...)` patterns

Fixes: #62
Fixes: #32
Partially Fixes: #24

"test-helper.el" isn't a library intended to be loaded with `require`.
Instead it is loaded with `load` by `ert-runner` itself.  Because of
that, it is confusing to provide a feature using `provide`.  It also
isn't possible to use `require' to load `test-helper` because that
might, depending on the order of `load-path` load the "test-helper.el"
of another package that used `ert-runner`.

This is also discussed at rejeep/ert-runner.el#38.