relation editor: use a public constructor, remove UI file

rename file according to class name
[custom widgets] add relation reference
QgsVectorLayer::uniqueValues() also returns uncommitted values
Followup 1bdef80: Fix crash when layer not in edit mode
allow reactivation of measurement after deactivation
Add icon size 48 and 64

To make life easier on touch devices
Fix #9527
[processing] fix vector functions definitions
check if value is None before trying to cast float

the tool used to fail when there were NULL values in the fields
Merge pull request #1564 from anitagraser/patch-2

[processing] check if value is None before trying to cast float