Refactored cursor and selection manipulation, and more! (#146)

* Refactored cursor and selection manipulation, and more!

Issue #145

textboxes.nim
- Implemented cursor + anchor model for selection
- Refactored almost all cursor movement procedurs into shiftCursor
- Created an Orient enum with enumerators for shiftCursor (e.g. Left,
NextWord, TheEnd)
- Created placeCursor procedure for direct placement by index
- Created clearSelection procedure
- Created growSelection procedure
- Rewrote delete and created deleteSelected
- Rewrote insert, cursorUp, cursorDown, `selection=`, and findLine
- Organized the cursor and selection procedures
- Wrote documentation for various procedures
- Change NextWord / PreviousWord functionality to always jump to the
beginning of each word

input.nim
- Ensured the cursor blinks
- Created activate() procedure
- Rewrote delete-previous-word logic
- Switched word skipping to Meta+Arrow-keys (this is much more common)
- Switched word selection to Meta+Shift+Arrow-keys
- Eliminated cursor alignment problem by connecting layoutResize slot

tinput.nim
- Ensured input is activated, and the cursor is visible on init
- Ensured cursor is placed at TheEnd on init

tinput2.nim
- Ensured input is activated, and the cursor is visible on init
- Ensured all numbers are editable

ttextboxes.nim
- Updated tests based on new model / procedures

* Update src/figuro/ui/textboxes.nim

* Update src/figuro/widgets/input.nim

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Co-authored-by: Jaremy Creechley <creechley@gmail.com>

Author: derek-v-s