Add arrow keys to move and resize selection rectangle

Summary:
Add arrow keys to move and resize selection rectangle.
Arrows alone moves in large increment
⇧ + Arrows moves in small (single pixel) increments
ALT + Arrows resize rectangle in large increment
ALT + ⇧ + Arrows resize rectangle in small increment

BUG: 394947

Test Plan:

• Apply patch
  • Launch Spectacle; begin Rectangular Selection
  • Select a rectangle with the mouse
  • Use Arrows to move rectangle (large increment)
  • Use ⇧ + Arrows to move in small increment
  • Use ALT + Arrows to resize rectangle (large increment
  • Use ALT + ⇧ + Arrows to resize rectangle in small increment
  • Complete capture with mouse or Enter key
  • Ensure right selection is captured after moving and resizing rectangle

Reviewers: rkflx, ngraham, Spectacle, yurchor

Reviewed By: rkflx, ngraham, Spectacle, yurchor

Subscribers: ltoscano, kde-doc-english, abalaji, Spectacle

Tags: Documentation

Differential Revision: https://phabricator.kde.org/D13450