QShortcut: replace 'clickFunc()' with lambda; rename variable to 'takeScreenshotShortcut'

Summary:
Removed `clickFunc()` entirely; replaced with lambda. Renamed `QShortcut` variable
to `takeScreenshotShortcut` for clarity. Eliminated redundant `QTimer::singleShot` function call,
as `animateClick()` emits all the necessary signals on its own.

Test Plan:
  - Launch Spectacle in GUI mode
  - Take screenshot by pressing {key Enter} (default GUI key)
  - Take screenshot by pressing {key CTRL}+{key N} ("new document" shortcut)

Reviewers: #spectacle, rkflx, ngraham

Reviewed By: #spectacle, rkflx

Differential Revision: https://phabricator.kde.org/D11374