Improvements of finance views

- updated search function: was displaying wrong month sometime, also
removed day field (was unused; still avaible in the comments though.)
- you can't search for a month in the future (this will give a warning
message.)
- updated approved month view in FinancesView
- updated actions when a month is being approved (directly disable
buttons)
- a month can not be approved when it contains no transactions (displays
a warning message)