- `cairomm` library added to support vector graphics
- Makefile (macOS) updated to support `cairomm`
- Callback/Handler function updated for drawing
- Example line drawing used from GNOME documentation
- BRK: This will break SemaphoreCI due to missing libs for build environment

- Layout now oriented vertically and the `display_area` will now be
drawn above the numberpad, like typical calculators.
- A C++ string container was added and now has the buttons appended to
it when they're pressed, and then emits the redraw signal to be enqueued
for the `display_area` widget.

- 4 rows of text elements created for the dyadic operations.
- Removed and cleaned-up commented-out code and copied comments.

- Added callback-handlers to write operators and change mode of the
calculator, based on a new `enum`, to determine which operator argument
to be writing button digits into.
- Strings update logically with button presses and 3/4s of the
calculator functionality is now in place.
- Equals button needs to added in next commit to add the solve function.

- "Solve" functionality added with the `=` button.
- Calculator can now solve basic algebraic field operations in any base
system from `0` to `16`.
- During testing a bug was discovered that the root selection enabling
of the buttons is actually uninitialized and the root selection itself
is enabling an extra button (off-by-1 indexing error).
- Simple examples tested and functionality seems to be working for the
`+`, `-`, and `*` operations of integers. Fractional values and division
have not yet been tested/verified.
- Current design is likely incompatible with fractional values, so it
may be more prudent to remove the decimal button from the UI for now.

- Slider (Range / Scale) fixed so it no-longer is off by one, though
unfortunately it now shows a fractional value.
- Calculator design currently only supports integer operations due to
string to int conversions, so the decimal point was removed to avoid
any potential bugs.