Improves datum transform selection. 

- Fixes issue with "Hide deprecated" check box
- Creates a score to sort multiple datum transform definitions
- Creates a entry on layers context menu to recall datum transform selection dialog
- Prevents multiple mouse clicks on "CRS status" icon on status bar
- Prevents settings update when project properties dialog is cancelled

Guess this can go - the message box below is commented out (otherwise I'd have said something about the missing `tr("...")` ;) )