Fix KP_2,4,6,8 shortcuts for rect tool

Merged Nathan Lee requested to merge nathanal/inkscape:inkscape_1270_2 into master

Also makes some functions const, and use them instead of accessing variable directly

Follow up to 999a24c3 Fix #1270 (closed)

Follow up on !3755 (merged), annoyingly I only noticed when I went to retest the issue before closing. Requires adding a new function.

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