Disable editing for multi-currency transactions
refuse to modify when saving
Put toast string to resource
Refuse to change account multi-currency transaction
Try to disable split edition for multi-currency transactions
Show correct transfer account
Keep currency for transaction splits
reject add split; show splits own currency
show correct split account
Merge branch 'develop' into dev-check-multi

Conflicts:
	app/src/org/gnucash/android/ui/transaction/dialog/SplitEditorDialogFragment.java
Merge pull request #228 from fefe982/dev-check-multi

Transaction involving multiple currencies cannot be modified.
Also add some bug fix to make mutli-currency transactions display correctly.

- Rewrite setSplits in Transaction. Now split is not added one by one through addSplit, but the split list is directly assigned. This way the currency information in split is kept. addSplit will reset the currency information. The currency information is used to display the correct currency symbol in split view.

- In TransactionFormFragment, when a multi-currency transaction is loaded, every control is disabled except the split edit button. Pressing "save" or changing the account dropdown list on the upper left corner wouldn't change the transaction, an warning is shown instead. Transfer account dropdown is loaded with accounts without the currency restriction, so the correct transfer account can be shown. The split edit button will bring the SplitEditDialogFragment.

- In SplitEditDialogFragment, when a multi-currency transaction is loaded, every control in each split is disabled. Pressing the "Add Split" button or "save" button would not change the transaction, a warning is shown instead. Balance in the lower right corner would not be calculated and fixed to zero. The account drop-down for each split is load without the currency restriction, so the correct account can be shown. As currency information for each split is kept (see 1), correct currency symbol is used for each split.